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A  HISTORY   OF   PHYSICS 


Jj^^^ 


A 

HISTOEY    OF    PHYSICS 

IN  ITS  ELEMENTARY  BRANCHES 

INCLUDING   THE 

EVOLUTION  OF  PHYSICAL  LABORATORIES 


BOSTON  COLLEGE 

FLORIAN   CAJORI,  PiffW^®'*^® 

Pbofesbor  of  the  History  of  Mathematics  in  the     ^  ^ 
CNiTEESiTr  of  California 


THE   MACMILLAN   COMPANY 

LONDON:  MACMILLAN  &  CO.,  Lt'^ 
1924 

All  rights  reserved 


COPYBIGHT,   1899, 

By  the  MACMILLAN  COMPANY. 


Set  up  and  electrotyped.     Published  January,  1899, 


NorJoooft  ^ress 

J.  8.  Gushing  tfe  Co.  —  Berwick  &  Smith  Co. 

Norwood,  Mass.,  U.S.A. 


PREFACE 

This  history  is  intended  mainly  for  tlie  use  of  students  and 
teachers  of  physics.  The  writer  is  convinced  that  some  atten- 
tion to  the  history  of  a  science  helps  to  make  it  attractive,  and 
that  the  general  view  of  the  development  of  the  human  intel- 
lect, obtained  by  reading  the  history  of  science,  is  in  itself 
stimulating  and  liberalizing. 

In  the  announcement  of  Ostwald's  Klassiker  der  ExaMen 
Wissenschaften  is  the  following  significant  statement:  "While, 
by  the  present  methods  of  teaching,  a  knowledge  of  science  in 
its  present  state  of  advancement  is  imparted  very  successfully, 
eminent  and  far-sighted  men  have  repeatedly  been  obliged  to 
point  out  a  defect  which  too  often  attaches  to  the  present 
scientific  education  of  our  youth.  It  is  the  absence  of  the  histor- 
ical sense  arid  the  ivant  of  knoivledge  of  the  great  researches  upon 
ivhich  the  edifice  of  science  rests.^^ 

It  is  hoped  that  the  survey  of  the  progress  of  physics  here 
presented  may  assist  in  remedying  this  defect  so  clearly 
pointed  out  by  Professor  Ostwald. 

As  it  seems  best  not  to  increase  the  size  of  the  book  beyond 
the  limit  originally  intended,  it  is  necessary  to  omit  a  few 
subjects  which  properly  belong  to  elementary  physics. 

It  gives  me  great  pleasure  to  acknowledge  my  obligations  to 
Mr.  S.  J.  Barnett,  Ph.D.,  and  Mr.  P.  E.  Doudna,  A.M.,  of  Col- 
orado College,  for  assistance  in  proof-reading  and  for  impor- 
tant suggestions  and  criticisms. 

FLORIAN  CAJOBI. 

Colorado  College,  Colorado  Springs, 
November,  1898. 

V 


CONTENTS 

PAGE 

The  Greeks 1 

Mechanics 1 

Light 6 

Electricity  and  Magnetism 8 

Meteorology 9 

Sound 10 

Atomic  Theory    . 12 

Causes  of  Failure  of  Greek  Physical  Inquiry     ....  13 

The  Romans 15 

The  Arabs 17 

Europe  during  the  Middle  Ages          ......  21 

Gunpoioder  and  the  Mariner''s  Compass 22 

Hydrostatics 25 

Light • 26 

The  Renaissance 27 

The  Copernican  System .  27 

Mechanics 30 

Light 37 

Electricity  and  Magnetism .        .41 

Meteorology 47 

The  Inductive  Method  of  Scientific  Inquiry        ....  48 

The  Seventeenth  Century .50 

Mechanics 50 

Light 76 

Heat 89 

Electricity  and  Magnetism 94 

Sound 97 

vii 


Vlll  CONTENTS 

PAGE 

The  Eighteenth  Centurt ,99 

Mechanics 99 

Light 101 

Heat 105 

Electncity  and  Magnetism '       .        .117 

Sound 136 

The  Nineteenth  Century 137 

Light  . 140 

Heat 189 

Electricity  and  Magnetism 215 

Sound ,        ,  279 

The  Evolution  of  Physical  Laboratories  .....    286 


A  HISTORY   OF  PHYSICS 


3>«C 


THE   GREEKS 

IiT  mathematics,  metaphysics,  literature,  and  art  the  Greeks 
displayed  wonderful  creative  genius,  but  in  natural  science 
they  achieved  comparatively  little.  It  would  not  be  correct 
to  say  that  they  possessed  little  or  no  aptitude  for  observing 
natural  phenomena,  but  it  is  true  that,  as  a  rule,  they  were 
ignorant  of  the  art  of  experimentation,  and  that  many  of  their 
physical  speculations  were  vague,  trifling,  and  worthless.  As 
compared  with  the  vast  amount  of  theoretical  deduction  about 
nature,  the  number  of  experiments  known  to  have  been 
performed  by  the  Greeks  is  surprisingly  small.  Littlejor 
no..-attempt  was_mja.dg_to  , verify  speculation  by  experimental 
eyideace.  As  a  conspicuous  example  of  misty  philosophizing 
we  give  Aristotle's  proof  that  the  world  is  perfect:^  ^'The 
bodies  of  which  the  world  is  composed  are  solids,  and  therefore 
have  three  dimensions.  Now,  three  is  the  most  perfect  number, 
—  it  is  the  first  of  numbers,  for  of  one  we  do  not  speak  as  a 
number,  of  two  we  say  both,  but  three  is  the  first  number  of 
which  we  say  all.  Moreover,  it  has  a  beginning,  a  middle, 
and  an  end." 

MECHANICS 

Mechanical  subjects  are  treated  in  the  writings  of  Aristotle. 
The  great  peripatetic  had  grasped  the  notion  of  the  parallelo- 

1  De  Ccelo,  I.  1,  as  translated  by  Whewell. 
1 


2  A    HISTORY   OF   PHYSICS 

gram  of  forces  for  the  special  case  of  the  rectangle.  He 
attempted  the  theory  of  the  lever,  stating  that  a  force  at  a 
greater  distance  from  the  fulcrum  moves  a  weight  more  easily 
because  it  describes  a  greater  circle.  He  resolved  the  motion 
of  a  weight  at  the  end  of  the  lever  into  tangential  and  normal 
components.  The  tangential  motion  he  calls  according  to  nature; 
the  normal  motion  contrary  to  nature.  The  modern  reader  will 
readily  see  that  the  expression  contrary  to  nature  applied  to  a 
natural  phenomenon  is  inappropriate  and  confusing. 

Aristotle's  views  of  falling  bodies  are  very  far  from  the  truth. 
Nevertheless  they  demand  our  attention,  for  the  reason  that, 
during  the  Middle  Ages  and  Eenaissance,  his  authority  was  so 
great  that  they  play  an  important  role  in  scientific  thought. 
He  says :  ''  That  body  is  heavier^ jthan  another  which,  in  an 
equal  bulk,  moves  downward  quicker."  ^  In  another  place  he 
teaches  that  bodies  fall_quicker  in  exact  proportion  to  their 


weight.^     No  statement  could  be  further  from  the  truth. 

A  modern  writer  endeavours  to  exonerate  Aristotle  as  a 
physicist.  "  If  he  could  have  had  any  modern  instrument  of 
observation  —  such  as  the  telescope  or  microscope,  or  even  the 
thermometer  or  barometer — placed  in  his  hands,  how  swiftly 
would  he  have  used  such  an  advantage ! "  ^  But  in  the  case  of 
falling  bodies,  the  experiment  was  within  his  reach.  If  it  had 
only  occurred  to  him,  while  walking  up  and  down  the  paths  near 

1  De  Ccelo,  IV.  1,  p.  308. 

2  This  law  is  assumed  by  him  in  the  following  reasoning :  "...  suppose 
a  without  weight,  but  /3  possessing  weight ;  and  let  a  pass  over  a  space  y8, 
but  /3  in  the  same  time  pass  over  a  space  ye,  — for  that  which  has  weight 
will  be  carried  through  the  larger  space.  If  now  the  heavy  body  be 
divided  in  the  proportion  that  space  ye  bears  to  yd,  .  .  .  and  if  the  lohole 
is  carried  through  the  whole  space  ye,  then  it  must  be  that  a  part  in  the 
same  time  would  be  carried  through  yd.  .  .  ." — De  Coelo,  Book  III., 
Ch.  II. 

3  Article  "Aristotle"  in  Encyclopaedia  Britannica,  Ninth  Edition. 


MECHANICS 


Ms  school  in  Athens,  to  pick  up  two  stones  of  unequal  weight 
and  drop  them  together,  he  could  easily  have  seen  that  the  one 
of,  say,  ten  times  the  wei^ht_dM^ot_d^scen^^^ 

Immeasurably  superior  to  Aristotle  as  a  student  of  mechanics 
is  Archimedes  (287(?)-212  b.c.).^  He  is  the  true  originator  of 
mechanics  as  a  science.  To  him  we  owe  the  theory  of  the 
^entre^  of^gravitj  (centroid)  and  of  the  lever.  In  his  Equi- 
ponderance  of  Planes  he   starts  with   the   axiom  that  equal 


Fig.  1. 


/^  weights  acting  at  equal  distances  on  opposite  sides  of  a 
\  pivot  are  in  equilibrium,  and  then  endeavours  to  establish  the 
\  principle  that  "  in  the  lever  unequal  weights  are  in  equilibrium 
only  when  they  are  inversely  proportional  to  the  arms  from 
which  they  are  suspended."  His  appreciation  of  its  efficiency 
^  is  echoed  in  the  exclamation  attributed  to  him :  "  Give  me  a 
fulcrum  on  which  to  rest,  and  I  will  move  the  earth." 

We  reproduce  from  a  mechanical  work  of  Varignon,  published 
in  Paris  in  1687,  a  figure  (Fig.  1)  illustrating  this  saying.  The 
Latin  motto  in  the  figure  may  be  rendered  thus:  "Touch  it 
and  you  will  move  it." 

1  Consult  The  Works  of  Archimedes,  edited  in  modern  notation,  with 
introductory  chapters,  by  T.  L.  Heath.     Cambridge,  University  Press. 


4  A   HISTORY   OF  PHYSICS 

While  the  Equipoyiderance  treats  of  solids  or  the  equilibrium 
of  solids,  the  book  on  Floating  Bodies  treats  of  hydrostatics. 
His  attention  was  first  drawn  to  the  subject  of  specific  gravity 
when  King  Hieron  asked  him  to  test  whether  a  crown,  professed 
by  the  maker  to  be  pure  gold,  was  not  alloyed  with  silver.  The 
story  goes  that  our  philosopher  was  in  a  bath  when  the  true 
method  of  solution  flashed  on  his  mind.  He  immediately  leaped 
from  the  bath  and  ran  home,  shouting,  "  I  have  found  it !  "  To 
solve  the  problem  he  took  a  piece  of  gold  and  a  piece  of  silver, 
each  weighing  the  same  as  the  crown.  According  to  one 
author,  ^  he  determined  the  volume  of  water  displaced  by 
the  gold,  silver,  and  crown  respectively,  and  calculated  from 
that  the  amount  of  gold  and  silver  in  the  crown.  According 
to  another  writer,^  he  weighed  separately  the  gold,  silver,  and 
crown,  while  immersed  in  water,  thereby  determining  their 
loss  of  weight  in  water.  From  these  data  he  easily  found  the 
solution.  It  is  possible  that  Archimedes  solved  the  problem 
by  both  methods. 

In  his  Floating  Bodies  Archimedes  established  the  important 
principle,  known  by  his  name,  that  the  loss  of  weight  of  a  body 
submerged  in  water  is  equal  to  the  weight  of  the  water  dis- 
placed, and  that  a  floating  body  displaces  its  own  weight  of 
water.  Since  the  days  of  Archimedes  able  minds  have  drawn 
erroneous  conclusions  on  liquid  pressure.  The  expression 
"hydrostatic  paradox "  indicates  the  slippery  nature  of  the 
subject.  All  the  more  must  we  admire  the  clearness  of  con- 
ception and  almost  perfect  logical  rigour  which  characterize  the 
investigations  of  Archimedes.^ 

1  VlTRUVIUS,  IX.  3. 

2  Scriptores  metrologici  Bomani  (ed.  Hultsch,  pp.  124-208). 

3  A  valuable  paper  with  numerous  extracts  from  authors  is  Ch. 
Thurot's  Becherches  Historiques  sur  le  Principe  cVArchimede,  Paris, 
1869  (extrait  de  la  Bevue  Archeolugique,  Aunees  1868-1869). 


MECHANICS  5 

Arcliimedes  is  said  to  have  shown  wonderful  inventive 
genius  in  various  mechanical  inventions.  It  is  reported  that 
he  astonished  the  court  of  Hieron  by  moving  heavy  ships  by 
aid  of  a  collection  of  pulleys.  To  him  is  ascribed  the  inven- 
tion of  war  engines,  and  the  endless  screw  ("  screw  of  Archi- 
medes ")  which  was  used  to  drain  the  holds  of  ships. 

About  a  century  after  Archimedes,  there  flourished  Ctesibius 
and  his  pupil  Heron,  both  of  Alexandria.  They  contributed 
little  to  the  advancement  of  theoretical  investigation,  but  they 
displayed  wonderful  mechanical  ingenuity.  The  force-pump 
is  proba,bly  the  invention  of  Ctesibius.  The  suction  pump  is 
older  and  was  known  in  the 
time  of  Aristotle.  According 
to  Yitruvius,  Ctesibius  de- 
signed the  ancient  fire-engine, 
consisting  of  the  combination 
of  two  force-pumps,  spraying 
alternately.  The  machine  had 
no  air-chamber,  and  therefore 
could  not  produce  a  steady 
stream.      Heron  describes  the  ^^^-  ^' 

fire-engine  in  his  Pneumatica.  During  the  Middle  Ages  the 
fire-engine  was  unknown.  It  is  said  to  have  been  first  used  in 
Augsburg  in  1518.^  Ctesibius  is  credited  with  the  invention  of 
the  hydraulic  organ,  the  water-clock,  and  the  catapult.  Heron 
showed  the  earliest  application  of  steam  as  a  motive  power,  in 
his  toy,  called  the  ^^eolipile"  (Fig.  2).  It  consisted  of  a  hollow 
sphere  with  two  arms  at  right  angles  to  its  axis  and  bent  in 
opposite  directions  at  its  ends.  When  steam  was  generated  in 
the  sphere,  it  escaped  through  the  arms  and  caused  the  sphere 
to  rotate.     It  was  the  forerunner  of  Barker's  water-mill  and  the 

1  A;  DE  RocHAS  in  La  Nature,  Vol.  XI.,  pp.  13,  14 ;  1883. 


6  A  HISTORY   OF   PHYSICS 

modern  turbine.     Heron  wrote  an  important  book  on  geodesy, 
called  Dioptra} 

The  Greeks  invented  the  hydrometer,  probably  in  the  fourth 
century  a.d.  There  appears  to  be  no  good  evidence  for 
attributing  its  origin  to  Archimedes.  The  hydrometer  is 
described  in  full  by  Bishop  Synesius  in  a  letter  to  Hypatia.  It 
consisted  of  a  hollow,  graduated,  tin  cylinder,  weighted  below. 
It  was  first  used  in  medicine,  to  determine  the  quality  of 
drinking-water,  hard  water  being  at  that  time  considered 
unwholesome.  According  to  Desaguliers  it  was  used  for  this 
purpose  as  late  as  the  eighteenth  century.^ 

LIGHT 

The  fragment  of  a  Greek  document,  found  in  Egypt,  speaks 
of  various  optical  illusions ;  for  instance,  tha,t  the  sun  appears 
larger  when  at  the  horizon  than  when  near  the  zenith.^  Optics 
is,  indeed,  one  of  the  oldest  branches  of  physics.  A  converging 
lens  of  rock  crystal  is  said  to  have  been  found  in  the  ruins  of 
Nineveh.''  In  Greece,  burning-glasses  seem  to  have  been  manu- 
factured at  an  early  date.  Aristophanes,  in  the  comedy  of  The 
Clouds,  Act  II.  (performed  424  b.c),  introduces  a  conversation 
about  "fine  transparent  stone  (glass)  with  which  fires  are 
kindled,"  and  by  which,  standing  in  the  sun,  one  can,  "though 
at  a  distance,  melt  all  the  writing"  traced  on  a  surface  of  wax. 
The  Platonic  school  taught  the  rectilinear  propagation  of  light 

1  For  a  full  account  of  Heron,  "the  first  engineer,"  see  W.  A.  Trues- 
DELL  in  Jour,  of  the  Ass.  of  Engin.  Soc,  Vol.  XIX.,  Philadelphia,  1897, 
pp.  1-19. 

2  E.  Gerland  in  Wiedemann'' s  Annalen,  Vol.  1,  New  Series^  1877,  pp. 
150-157.     See  also  his  Gesch.  d.  Physik,  p.  40. 

3  See  K.  Wessely  in  Wiener  Studien,  Vol.  13,  1891,  pp.  312-323. 
Abstracted  in  Wiedemann'' s  Beihldtter,  Vol.  17,  1893. 

*  E,  Gerland,  Geschichte  der  JP\ysik,  Leipzig,  1892,  p.  9. 


LIGHT 


and  the  equality  of  the  angle  of  incidence  to  that  of  reflection. 
The  astronomer,  Claudius  Ptolemy,  who  flourished  in  Alex- 
andria in  139  A.D.,  measLired  angles  of  incidence  and  of 
refraction,  and  arranged  them  in  tables. 

Metallic  mirrors  seem  to  have  been  manufactured  in  re- 
mote antiquity.  ''  Looking-glasses  "  are  referred  to  in  Exodus 
38 : 8,  and  in  Joh  37 :  18 ;  they  have  been  found  in  graves  of 
Egyptian  mummies.  Spherical  and  parabolic  mirrors  were 
known  to  the  Greeks.  To  Euclid  (about  300  b.c.)  is  attributed 
a  work  on  Catoptrics,  dealing  with  phenomena  of  reflection. 
In  it  is  found  the  earliest  reference  to  the  focus  of  a  spherical 
mirror.  In  Theorem  30  it  is  stated^  that  concave  mirrors 
turned  toward  the  sun  will  cause  ignition.  In  the  ^' frag- 
mentum  Bobiense,"  a  document  written,  perhaps,  by  Anthe- 
mius  of  Tralles,  the  focal  property  of  parabolic  reflectors  is 
demonstrated.  Several  Greek  authors  appear  to  have  written 
on  concave  mirrors.  The  story  that,  when  the  Eomans  were 
besieging  Syracuse,  Archimedes  defended  his  native  city  by 
the  use  of  mirrors  reflecting  the  sun's  rays,  and  setting  on  fire 
the  ships  when  they  came  within  bowshot  of  the  walls,  is 
probably  a  fiction. 

The  Greeks  elaborated  several  theories  of  vision.  According 
to  the  Pythagoreans,  Democritus,  and  others  vision  is  caused 
by  the  projection  of  particles  from  the  object  seen,  into  the 
pupil  of  the  eye.  On  the  other  hand,  Empedocles  (about  440 
B.C.);  the  Platonists,  and  Euclid  held  the  strange  doctrine  of 
ocular  beams,  according  to  which  the  eye  itself  sends  out 
something  which  causes  sight  as  soon  as  it  meets  something 
else  emanated  by  the  object.^ 

1  Euclidis  Opera  Omnia,  Vol.  7,  Edidit  I.  L,  Heiberg,  Lipsise,  1895. 
See  also  E.  Wiedemann  in  Wied.  Annalen,  Vol.  39,  1890,  p.  123. 

2  For  Plato's  theory,  see  The  Dialogues  of  Plato,  Vol.  II.,  translated 
by  B.  JowETT,  C.  Scribner's  Sons,  New  York,  pp.  537  et  seq. 


8  A   HISTORY   OF   PHYSICS 


ELECTEICITY  AND  MAGNETISM 

To  the  Greeks  we  owe  a  few  isolated  observations  on  elec- 
tricity and  magnetism.  TJiales  of  Miletus  (640-546  b.c),  one 
of  the  "  seven  wise  men  "  of  early  Greece,  is  credited  with  the 
knowledge  that  amber,  when  rubbed,  will  attract  light  bodies, 
and  that  a  certain  mineral,  now  called  magnetite,  or  loadstone, 
possesses  the  power  of  attracting  iron.  Amber  —  a  mineral- 
ized yellowish  resin  —  was  used  in  antiquity  for  decoration. 
In  common  with  the  bright  shining  silver-gold  alloys,  and  gold 
itself,  it  was  called  "  electron  "  ;  hence  the  word  "  electricity.'^ 
About  three  centuries  after  Thales,  Theophrastus,  in  his  treatise 
On  Gems,  mentions  another  mineral  which  becomes  electrified 
by  friction.  We  know  now  that  all  bodies  can  be  thus  elec- 
trified. Pliny  says  that  ignorant  people  called  the  loadstone 
"  quick-iron."  The  large  extent  to  which  this  phenomenon  of 
magnetic  attraction  excited  the  imagination  of  men  is  shown 
by  the  fable  of  the  shepherd  Magnes,  who,  on  Mount  Ida  (on 
the  island  Crete),  was  so  strongly  drawn  to  earth  by  the  tacks  in 
his  sandals  and  the  iron  tip  of  his  staff,  that  he  could  hardly 
pull  himself  away.  He  dug  to  ascertain  the  cause,  and  dis- 
covered a  wonderful  stone  (magnetite).  Another  fable  speaks 
of  a  powerful  magnetic  mountain,  which  pulled  the  nails  out 
of  ships,  even  when  the  latter  were  at  considerable  distance 
from  it.^ 

Pliny  tells  another  story  concerning  the  loadstone.  At 
Alexandria  the  construction  of  a  vaulted  roof  of  magnetite 
in  the  temple  of  Arsinoe  was  undertaken  for  the  purpose 
of  suspending  in  the  air  the  iron  statue  of  the  queen.  As  time 
went  on,  the  story  was  greatly  embellished.     Thus,  according 

1  This  story  recurs  frequently  in  literature  ;  for  instance,  in  the  tale  of 
the  third  mendicant  in  the  Arabian  Nights. 


METEOROLOGY  9 

to  the  Venerable  Bede,  the  horse  of  Bellerophon,  on  the  island 
of  Ehodes,  weighed  5000  pounds,  and  was  suspended  by  mag- 
nets.^ A  similar  story  is  told  of  Mohammed's  coffin.  Of  course, 
such  a  suspension  in  air  is  mechanically  impossible. 

During  antiquity  iron  was  mined  chiefly  along  the  coasts  of 
the  ^gean  Sea  and  on  the  Mediterranean  islands.  Magnetic 
iron  ore  is  said  to  have  been  found  also  near  Magnesia  in  Asia 
Minor.  According  to  Lucretius  the  term  "  magnet "  is  derived 
from  "  Magnesia."  There  were  iron  mines  on  the  island  of 
Samothrace.  The  miners  of  that  locality  showed  the  action 
of  the  loadstone  in  connection  with  the  so-called  Samothracian 
rings.  Says  Socrates :  ".  .  .  that  stone  not  only  attracts  iron 
rings,  but  also  imparts  to  them  a  similar  power  of  attracting 
other  rings ;  and  sometimes  you  may  see  a  number  of  pieces  of 
iron  and  rings  suspended  from  one  another  so  as  to  form  quite 
a  long  chain :  and  all  of  them  derive  their  power  of  suspension 
from  the  original  stone."  ^ 

The  polarity  of  magnets  and  the  phenomenon  of  repulsion 
which  may  exist  between  electric  charges  or  magnetic  poles 
were  unknown  to  Greek  antiquity. 

METEOEOLOGY 

Previous  to  the  middle  of  the  fifteenth  century  no  syste- 
matic meteorological  records  are  known  to  have  been  kept 
anywhere.^  Yet  the  Greeks  paid  some  attention  to  meteo- 
rology. It  is  in  Athens  that  we  find  the  oldest  contrivance 
for  observing  the  direction  of  the  wind.    There,  in  its  essential 

1  Beda,  De  Sept.  Mirac.  Mundi;  quoted  by  Park  Benjamin  in  The 
Intellectual  Bise  in  Electricity^  New  York,  1895,  p.  46.  (Hereafter  this 
work  will  be  referred  to  as  Benjamin.) 

2  JowETT,  Dialogues  of  Plato,  Vol.  I.,  p.  223.     (Ion.) 

«  G.  Hellmann,  Himmel  und  Erde^  Vol.  II.,  1890,  p.  113. 


10  A   HISTORY    OF   PHYSICS 

parts  standing  to  this  day,  is  the  "  tower  of  the  winds/'  built 
about  100  B.C.  Upon  an  octagon  of  marble  was  a  roof,  the 
highest  part  of  which  carried  a  weather-vane  in  form  of  a 
triton.  It  is  improbable  that  weather-vanes  were  ever  com- 
mon in  Greece  or  Eome,  for  there  is  no  Greek  or  Latin  name 
to  designate  the  instrument.^  Among  the  Greeks  meteorology 
can  hardly  be  said  to  have  risen  to  the  dignity  of  a  science. 
Theophrastus  of  Eresus  (371-286  B.C.),  a  disciple  of  Aristotle, 
wrote  a  book  On  Winds  and  on  Weather  Signs,^  but  like  most 
other  Greek  philosophers,  he  was  hardly  the  man  to  adopt 
patient  and  exact  observation  in  place  of  dogmatic  assertion 
and  the  teaching  of  authority.  Aristotle  makes  a  good  observa- 
tion on  the  formation  of  dew ;  viz.  dew  is  formed  only  on  clear 
and  quiet  nights.^ 

Aratus  of  Soli,  who  lived  about  275  b.c,  wrote  a  book  of 
Prognostics,  giving  predictions  of  the  weather  from  observation 
of  astronomical  phenomena,  and  various  accounts  of  the  effect 
of  weather  on  animals.  Several  editions  of  this  and  other 
works  of  Aratus  were  printed;  one  edition  was  brought  out 
by  Melanchthon. 

SOUND 

The  pyramids  of  Egypt  and  the  ruins  of  ancient  cities  bear 
testimony  to  the  fact  that  practical  geometry  and  practical 
mechanics  antedated  by  many  centuries  the  earliest  records 
which  we  possess  on  abstract  geometry  and  theoretical  me- 
chanics. In  the  same  way,  the  knowledge  of  vocal  and  instru- 
mental music,  said  to  have  been  possessed  by  nations  of  great 

1  Hellmann,  op.  cit.,  p.  119. 

^  Translated  by  J.  G.  Wood,  London,  1894,  with  an  introduction  and 
an  appendix  of  historical  interest  and  value. 

3  J.  C.  PoGGENDORFF,  GescMchte  der  Physik,  Leipzig,  1879,  p.  42. 
(Hereafter  this  work  will  be  quoted  as  Poggendorff.) 


SOUND  11 

antiquity,  demonstrates  that  the  art  of  music  is  incomparably 
older  than  the  theory  of  acoustics.  The  beginning  of  the 
theory  of  harmonics  reaches  back  to  Pythagoras  (580  ?-500  ? 
B.C.),  but  the  accounts  of  his  researches  are  so  intertwined 
with  fable  and  with  error,  that  it  is  difficult  to  ascertain  just 
what  Pythagoras  did.  Passing  by  a  blacksmith's  shop,  he  is 
said  to  have  noticed  that  the  hammers  as  they  struck  the  anvil 
produced  sounds  having  the  intervals  a  "  fourth,"  a  "  fifth," 
and  an  "octave."  He  found  the  weights  of  the  hammers^  to 
be,  respectively,  as  1 :  J  :  -| :  |-.  Subsequent  experimentation 
with  musical  strings  of  the  same  material  and  equal  lengths 
and  thicknesses  showed  that  weights  proportionate  to  1,  f ,  f,  ^, 
would  give  the  above  intervals.  This  research  pointed  to  an 
arithmetical  relation  between  musical  intervals,  and  estab- 
lished a  close  connection  between  subjects  so  far  apart  as 
arithmetic  and  music. 

It  will  readily  be  seen  that  the  above  account  contains  two 
errors.  Hammers  of  the  weights  given  above  will  not  yield 
the  sounds  in  question.  Nor  is  the  law  of  weights  for  strings 
stated  correctly ;  the  pitch  of  tones  varies,  not  as  the  weights, 
but  as  the  square  roots  of  the  weights. 

Some  modern  writers  have  been  led  to  surmise  that  Pythag- 
oras did  not  base  his  opinions  upon  experiment,  that  the 
smithy  in  which  he  got  his  information  was  the  land  of  Egypt, 
whence  he  imported  his  knowledge.^  Other  writers  assume 
that  Pythagoras  really  did  not  vary  the  tensions  of  the  strings, 
but  varied  their  lengths,  thereby  arriving  at  the  correct  law 
that  pitch  changes  inversely  as  the  lengths  of  the  strings.^    It 

iNicoMACHFS,  Harmonices,  I.,  p.  10  (Ed.  Meiboraius) ;  Porphyry, 
Ptol.  Harm.,  c.  3,  p.  213  ;  Diogenes  Laertius,  VIII.,  12. 

■^  See  article  "Music"  in  Encycl.  Brit.,  9th  ed.  This  article  contains 
much  information  on  Greek  musical  scales. 

^  Helmholtz,  Sensations  of  Tone,  trans,  by  A.  J.  Ellis,  London,  1885, 


12  A   HISTORY   OF   PHYSICS 

is  said  that  Pythagoras  was  the  first  to  establish  the  eight 
complete  degrees  in  the  diatonic  scale. ^ 

His  speculations  on  harmony  and  musical  intervals  were 
uncontrolled  by  further  inquiry  into  the  facts.  The  seven 
planets  are  the  seven  strings  of  the  lyre,  which  give  us  a 
beautiful  "harmony  of  the  spheres."^  This  idea  was  not 
advanced  as  poetry,  but  as  physical  philosophy.  The  fact  that 
the  human  ear  cannot  detect  such  interplanetary  music  did  not 
seem  to  weaken  his  belief  in  its  existence  ! 

The  theory  of  sound  was  touched  upon  by  Aristotle,  who 
entertained  correct  ideas  on  the  character  of  the  motion  of  air 
constituting  sound,  and  who  knew  that,  if  the  length  of  a  pipe 
is  doubled,  a  vibration  in  it  occupies  double  the  time. 

ATOMIC   THEOE^Y 

It  is  worthy  of  notice  that  the  atomic  theory  finds  its 
earliest  advocates  in  Greece.  That  the  theory  of  the  atomic 
constitution  of  matter  is  far  from  being  a  self-evident  truth 
follows  at  once  from  the  fact  that  the  two  thinkers  who  have 
swayed  philosophic  thought  most  powerfully,  Aristotle  and 
Kant,  teach  that  space  is  continuously  filled.^  The  great 
ancient  expositor  of  the  atomic  theory  is  Democritus  of  Abdera 
(about  460-370  e.g.).  He  taught  that  the  world  consists  of 
p.iinjvhy_s]2a.c.^  pnr]^ an  infinite  number  of  indivisible,  invisibly 
small  atoms.     Bodies  appear  and  disappeaF"MiylTfThe  union 

p.  1.  For  fuller  references  and  details  regarding  Pythagoras,  see  E.  Zel- 
LER,  History  of  Greek  Philosophy,  trans,  by  S.  P.  Alleyne,  London,  1881, 
Vol.  I.,  pp.  431-433.  Consult  also  C.  H.  H.  Parry,  The  Evolution  of  the 
Art  of  Music,  New  York,  1896,  "Scales,"  pp.  15-47. 

1  Helmholtz,  op.  cit.,  p.  266. 

2  NicoMACHus,  op.  cit.,  I.,  p.  6,  XL,  p.  33  ;  Pliny,  H.  N.,  II.,  p.  20 ;  SimpL 
in  Arist.  de  Ccelo.  Schol.,  p.  496,  11, 

3  Kurd  Lasswitz,  Geschichte  der  Atomistik,  Vol.  I.,  p.  2- 


FAILUBE   OF   GEEEK  PHYSICAL   CULTtTRE  IS 

and  separation  of  atoms.  Even  the  phenomena  of  sensation 
and  thought  are  the  result  of  their  combination.  The  atomic 
theory  did  not  play  any  great  role  in  scientific  progress  until 
after  the  discovery  by  Dalton  of  the  chemical  lawof^ultiple 
proportions. 

CAUSES   OF  THE  FAILUEE   OF   GEEEK   PHYSICAL 

INQUIRY 

While  the  Creeks  achieved  more  in  physical  research  than 
did  other  nations  of  antiquity,  they  nevertheless  accomplished 
infinitely  less  in  this  field  of  intellectual  activity  than  in  other 
directions.  The  question  why  the  Greeks  made  no  progress  in 
physics  is  an  old  puzzle,  and  is  not  easily  answered.  Francis 
Bacon  says  that  "  the  proceeding  has  been  to  fly  at  once  from 
the  sense  and  particulars  up  to  the  most  general  propositions,  as 
certain  fixed  poles  for  the  argument  to  turn  upon,  and  from 
these  to  derive  the  rest  by  middle  terms :  a  short  way,  no 
doubt,  but  precipitate;  and  one  which  will  never  lead  to 
nature,  though  it  offers  an  easy  and  ready  way  to  disputation.'' 
"  The  ancients  proved  themselves  in  everything  that  turns  on 
wit  and  abstract  meditation,  wonderful  men."  ^  According  to 
Whewell,  "  the  defect  was,  that  though  they  had  in  their  pos- 
session facts  and  ideas,  the  ideas  were  not  distinct  and  appro- 
priate to  tJiefacts.'^^  Consider,  for  example,  Aristotle's  motions 
^'  according  to  nature,"  and  "  contrary  to  nature,"  attributed  to 
the  lever.  Neither  Bacon's  nor  Whewell's  explanation  seems 
satisfactory.  Each  endeavours  to  explain  Jiow  the  thing  took 
place,  rather   than  why.     The   question   still   remains   to   be 

1  F.  Bacon,  in  Preface  to  the  "  Novum  Organum  "  (  Works,  New  York, 
1878,  Vol.  I.,  pp.  42,  32.) 

2  W.  Whewell,  History  of  the  Inductive  Sciences,  New  York,  1858, 
Vol.  I.,  p.  87.     (To  be  quoted  hereafter  as  Whewell.) 


14  A  HISTORY   OF   PHYSICS 

answered,  why  did  people  of  such  great  penetration  "fly  at 
once  from  the  sense  and  particulars  up  to  the  most  general 
propositions,"  or  why  did  they  come  to  apply  ideas  "not 
distinct  and  appropriate  to  the  facts "  ?  What  causes  led 
keen  minds  thus  to  blunder  ?  Perhaps  a  more  satisfactory 
answer  is  given  in  Mill's  System  of  Logic :  The  Greeks  "  were 
not  content  merely  to  know  that  one  phenomenon  was  always 
followed  by  another ;  they  thought  that  they  had  not  attained 
the  true  aim  of  science,  unless  they  could  perceive  something  in 
the  nature  of  the  one  phenomenon,  from  which  it  might  have 
been  known  or  presumed  previous  to  trial,  that  it  would  be 
followed  by  the  other.  .  .  .  [They]  not  only  sought  for  causes 
which  should  carry  in  their  mere  statement  evidence  of  their 
efficiency,  but  fully  believed  that  they  had  found  such  causes."  ^ 
"  When  Thales  and  Hippo  held  that  moisture  was  the  universal 
cause  and  eternal  element,  of  which  all  other  things  were  but 
the  infinitely  various  sensible  manifestations  ;  ^  when  Anaxime- 
nes  predicated  the  same  thing  of  air,^  Pythagoras  of  numbers, 
and  the  like,  they  all  thought  that  they  had  found  a  real 
explanation,  and  were  content  to  rest  in  this  explanation  as 
•  ultimate."  * 

1  J.  S.  Mill,  System  of  Logic,  London,  1851,  Vol.  I.,  p.  367,  where 
Mill  adopts  the  view  of  a  writer  in  the  Prospective  Beview,  February, 
1850. 

2  Moisture  is  the  necessary  constituent  of  food  ;  it  is  essential  to  germi- 
nation ;  the  fertility  of  land  depends  on  it. 

3  All  creatures  breathe  air,  live  on  it,  and  lastly,  are  transformed  into  it. 
*  Besides  the  works  already  quoted,  the  reader  interested  in  Greek 

science  may  consult  August  Heller,  Geschichte  der  Physik,  Stutt- 
gart, 1882,  Vol.  I.,  pp.  1-157 ;  G.  Milhaud,  Origines  de  la  Science 
Grecque,  Paris,  1893. 


THE  ROMANS 

The  genius  of  the  Roman  people  was  exercised  in  war,  con- 
quest, government,  and  law,  but  no  effort  was  put  forth  for  the 
advancement  of  pure  mathematics  or  science.  The  Homan 
scientific  writers  were  contented  to  collect  the  researches  of 
Greek  predecessors.  Among  these  are  Marcus  Vitruvius 
Pollio  (85-26  B.C.),  the  architect  of  Emperor  Augustus ;  Titus 
Cams  Lucretius  (95-52  (?)  b.c),  the  author  of  De  Rerum 
Natura;  Lucius  Annceus  Seneca  (2-66  a.d.),  the  tutor  of 
Emperor  Nero;  Pliny  (23-79  a.d.),  the  compiler  of  a  large 
work  on  natural  history;  and  Anicius  Manlius  Severinus 
Boethius  (480  ?-524),  at  one  time  a  favourite  of  King  Theodoric. 

Boethius  wrote  a  work  on  Music  which  contains  much  infor- 
mation on  Greek  theories  of  harmony.  Seneca  taught  the 
identity  of  rainbow  colours  with  those  formed  by  the  edge  of 
a  piece  of  glass.  He  observed  that  a  globular  glass  vessel, 
filled  with  water,  magnifies  objects,  but  he  was  led  by  this 
observation  no  further  than  to  remark  that  nothing  is  so 
deceptive  as  our  sight.  His  writings  are  replete  with  moral 
sentiment.  This  accounts,  perhaps,  for  the  fact  that  his 
Naturalium  quaestionum  lihri  VII  was  used  for  so  long,  during 
the  Middle  Ages,  as  a  text-book  of  physics.^  His  grasp  of 
mechanics  is  illustrated  by  the  story  which  he  gravely  tells 
of  a  fish,  less  than  a  foot  long,  which,  by  clinging  to  a  ship, 
completely  stops  its  motion  even  in  a  gale.     He  claimed  that, 

^  F.  RosENBEEGEE,  GescMchte  der  PhysiJc,  Part  I.,  1882,  p.  45.  (This 
work  will  be  quoted  after  this  as  Rosenbeegee.) 

15 


16  A   HISTORY   OF   PHYSICS 

during  the  battle  of  Actium,  Antonius's  largest  vessel  was 
thus  bound  fast. 

Cleomedes,  whose  place  and  time  of  birth  are  unknown, 
probably  flourished  about  the  time  of  the  Emperor  Augustus. 
He  noticed,  as  did  Archimedes  and  Euclid,  that  a  ring  on  the 
bottom  of  an  empty  vessel,  just  hidden  by  the  edge,  becomes 
visible  when  the  vessel  is  filled  with  water.  But  he  goes 
further  and  suggests  that  in  the  same  way  the  sun  may  be  in 
sight  when,  as  a  matter  of  fact,  it  is  a  little  below  the  horizon. 
Thus  he  is  the  first  to  consider  atmospheric  refraction. 

Lucretius  is  the  first  ancient  writer  who  refers  to  the  repul- 
sive effect  of  a  magnet  and  to  the  experiment  with  iron  filings. 
The  latter  "  will  rave  within  brass  basins,"  when  the  loadstone 
is  placed  beneath. 


THE  AEABS 

The  growth  of  the  Arabic  nation  presents  an  extraordinary 
spectacle  in  intellectual  history.  Scattered  barbaric  tribes 
were  suddenly  fused  in  the  furnace  blast  of  religious  enthu- 
siasm into  a  powerful  nation.  A  career  of  war  and  conquest 
was  followed  by  a  period  of  intellectual  activity.  About  the 
eighth  century  a.d.  the  Mohammedans  began  to  figure  as  the 
intellectual  leaders  of  the  world.  With  wonderful  celerity 
they  acquired  the  scientific  and  philosophic  treasures  of  the 
Hindus  and  Greeks.  Old  books  were  translated  from  the 
Greek  into  Arabic.  Chemistry,  astronomy,  mathematics,  and 
geography  became  favourite  subjects  of  study.  In  a  few 
instances  the  Arabs  made  original  contributions  to  science, 
but  as  a  rule  they  did  not  distinguish  themselves  in  original 
research ;  they  were  learned  rather  than  creative. 

So  far  as  we  know,  there  was  only  one  branch  of  physics 
which  was  successfully  cultivated  on  Arabic  soil  and  but  one 
man  prominently  identified  with  it.  The  branch  was  optics, 
and  the  man  was  Al  Hazen  (965? -1038).  His  full  Arabic 
name  is  Abu  'Ali  al  Hasan  ibn  al  Hasan  ibn  Al  Haitam.  He 
was  born  in  Bosra  oil  the  Tigris  and  rose  to  the  position  of 
vizier.  He  was  then  called  to  Egypt  by  one  of  the  caliphs 
who  had  heard  that  Al  Hazen  had  thought  out  plans  for  so 
regulating  the  flow  of  the  Nile  that  each  year  there  should 
be  plenty  of  water  for  irrigation.  Closer  inspection  of  the 
grounds  compelled  him  to  abandon  the  project.  He  committed 
other  errors  which  brought  him  into  disfavour  with  the  caliph. 

17 


18  A   HISTORY   OF   PHYSICS 

He  feigned  insanity  and  sought  concealment  until  after  tlie 
death  of  the  caliph.  Subsequently  he  made  his  living  by 
copying  manuscripts.  He  wrote  on  astronomy,  mathematics, 
and  optics. 

His  Ojytics  was  translated  into  Latin  and  printed  at  Bale  in 
1572.  To  the  law  of  the  equality  of  the  angles  in  reflection, 
which  he  learned  from  the  Greeks,  he  added  the  law  that  both 
angles  lie  in  the  same  plane.  He  made  a  study  of  spherical 
and  parabolic  mirrors.  The  greater  the  number  of  rays  which 
pass  through  a  point,  the  more  intense  is  the  heat  there.  Rays 
incident  upon  a  spherical  mirror,  and  parallel  to  the  principal 
axis,  are  reflected  to  this  axis.  All  the  rays  reflected  from 
points  in  the  mirror  lying  on  the  circumference  of  a  circle 
which  is  perpendicular  to  the  axis  (and  these  rays  only)  pass 
through  one  and  the  same  point  on  the  axis.  He  constructed 
a  mirror  out  of  a  number  of  separate  spherical  rings,  of  which 
each  has  its  own  radius  and  its  own  centre,  but  so  chosen  that 
all  rings  reflect  all  the  rays  accurately  to  one  and  the  same 
point.  The  following  is  known  as  "Al  Hazen's  problem": 
Given  the  position  of  a  luminous  point  and  of  the  eye,  to  find 
the  point  on  the  spherical,  cylindrical,  or  conical  mirror  at 
which  the  reflection  takes  place.  The  beginnings  of  this 
problem  are  found  in  Ptolemy's  optics  ;  after  Al  Hazen's 
masterly  but  complicated  discussion  of  it,  it  became  famous  in 
Europe  on  account  of  the  geometrical  difliculties  to  which  the 
general  problem  gave  rise.^ 

In  repetition  of  what  had  been  done  by  Ptolemy,  Al  Hazen 

1  For  Al  Hazen  and  his  researches  see  Paul  Bode,  "Alhazensche 
Spiegel- Aufgabe,"  Separat-Abdmck  aus  dem  Jahresbericht  des  Physi- 
kalischen  Vereins  zu  Frankfurt  a.  M.,  1891-92  ;  Leopold  Schnaase,  Die 
Optik  Alhazens,  Pr.  Stargard,  1889 ;  Baarman  in  Zeitschr.  d.  deutschen 
Morgenl.  Gesellschaft,  36,  1882,  p.  195  ;  E.  Wiedemann,  in  Wiedemann's 
Annalen,  N.  P.,  Vol.  39,  pp.  110-130  ;  also  Vol.  7,  p.  680. 


THE  ARABS  19 

took  measurements  of  angles  of  incidence  and  of  refraction, 
but,  like  his  predecessor,  lie  failed  to  discover  the  true  law  of 
refraction.  His  apparatus  consisted  of  a  graduated  circular 
copper  ring,  supported  in  a  vertical  position,  and  dipped  half 
way  into  water.  The  incident  ray  passed  through  a  hole  in 
the  rim  of  the  ring  and  through  a  perforated  disk  at  the  centre. 
The  apparatus  closely  resembles  that  utilized  at  the  present 
time  in  elementary  instruction,  and  has  the  great  advantage  of 
permitting  the  angles  of  incidence  and  refraction  to  be  read 
directly. 

The  apparent  increase  in  diameter  of  sun  and  moon,  when 
near  the  horizon,  he  declares  to  be  an  illusion  due  to  the  fact 
that  their  size  is  estimated  by  that  of  the  less  distant  ter- 
restrial objects.  This  explanation  has  held  its  ground  to  the 
present  day,  but  is  not  accepted  by  all.  Al  Hazen  arrived  at 
the  conclusion  that  the  planets  and  fixed  stars  do  not  receive 
their  light  from  the  sun,  but  are  self-luminous.^ 

Al  Hazen  is  the  first  physicist  to  give  a  detailed  description 
of  the  human  eye.  He  says  that  he  took  his  account  from 
works  on  anatomy.  Some  of  his  Arabic  predecessors  and  con- 
temporaries, as  well  as  he  himself,  stoutly  combat  the  theory 
of  Euclid  and  the  Platonists,  that  vision  is  due  to  rays  given 
out  by  the  eye ;  they  supported  the  view  of  Democritus  and 
Aristotle  that  the  cause  of  vision  proceeds  from  the  object 
seen.^ 

The  Arabs  developed  the  notion  of  "  specific  gravity,"  and 
gave  experimental  methods  for  its  determination.  Al  Biruni 
used  for  this  purpose  a  vessel  with  a  spout  slanting  down- 
wards.    It  was  filled  with  water  up  to  the  spout,  then  the  solid 


1  His  paper  on  this  subject  is  published  in  German  translation  by 
E.  Wiedemann  in  Wochenschr.  f.  Astr.,  Meteor.,  u.  Geogr.,  1890,  No.  17. 

2  B.  Wiedemann  in  Wied.  Annalen,  Vol.  39,  1890,  p.  470. 


20  A   HISTORY   OF   PHYSICS 

was  immersed,  and  the  weight  of  the  overflow  determined. 
This,  together  with  the  weight  of  the  solid  in  air,  yielded  the 
specific  gravity.  Al  Khazini,  in  his  Book  of  the  Balance  of 
Wisdom,  written  in  1137,^  describes  a  curious  beam  balance, 
with  five  pans,  for  weighing  in  air  and  in  water.  One  pan  was 
movable  along  the  graduated  beam.  He  points  out  that  air, 
too,  must  exert  a  buoyant  force,  causing  bodies  to  weigh  less.  ^ 

1  Extracts  are  translated  in  Journal  of  American  Oriental  Society, 
VI.,  pp.  1-128;  consult  also  F,  Kosenberger,  Part  I.,  pp.  81-86. 

2  Readers  interested  in  water-clocks  among  the  Arabs  may  consult 
A.  WiTTSTEiN,  "  Ueber  die  Wasseruhr  und  das  Astrolabium  des  Arza- 
chel,"  in  ScMomilch's  Zeitschr.,  Vol.  39,  1894,  Hist.  Lit.  Abtheilung,  p.  43, 


EUROPE  DURING  THE  MIDDLE  AGES 

With  the  third  century  of  onr  era  there  began  a  migration 
of  barbaric  nations  in  Europe.  The  powerful  Goths  from  the 
north  swept  onward  in  a  southwesterly  direction,  crossing 
into  Italy  and  shattering  the  Roman  Empire.  The  Dark  Ages 
which  followed  were  the  germinating  season  of  the  institutions 
and  nations  of  Europe.  Christianity  was  introduced,  and 
Latin  became  the  language  of  intercourse  in  ecclesiastical  and 
learned  circles. 

Obscurity  and  servility  of  thought,  indistinctness  of  ideas, 
and  mysticism  characterize  the  Middle  Ages.  Writers  on  sci- 
ence were  mainly  commentators,  and  never  thought  of  bringing 
the  statements  of  ancient  authors  to  the  test  of  experiment. 
At  first  the  science  of  the  Middle  Ages  was  drawn  largely 
from  Latin  sources.  The  insignificance  of  Roman  science  has 
been  already  pointed  out.  But  Eoman  writers  frequently  refer 
to  Greek  authors,  and  the  desire  naturally  arose  to  read  Greek 
authors  directly.  This  craving  was  partly  satisfied  by  the 
acquisition,  in  the  twelfth  century,  of  Arabic  translations  of 
Greek  treatises.  The  writings  of  Aristotle  became  well  known 
and  began  to  assume  supreme  authority.  Woe  unto  him  who 
dared  to  contradict  a  statement  made  by  Aristotle !  Witness 
Petrus  Ramus  (1515-1572),  who  in  Paris  was  forbidden  on 
pain  of  corporal  punishment  to  teach  or  write  against  the 
great  philosopher.  In  physics,  Aristotle's  authority  remained 
unshaken  until  the  time  of  Galileo. 

21 


22  A   HISTORY    OF   PHYSICS 

GUNPOWDER  AND  MARINER'S   COMPASS 

The  Europeans  of  this  period  came  into  possession  of  two 
inventions  which  have  greatly  influenced  the  progress  of  civili- 
zation, viz.  gunpowder  and  the  compass.  Their  origin  is 
shrouded  in  darkness.  The  preparation  of  gunpowder  out  of 
sulphur,  saltpetre,  and  charcoal  was  known  to  Marcus  Gtcbcus 
in  the  eighth  (?)  century,  and  to  Albertus  Magnus  about  1250. 
It  is  said  to  have  been  used  in  Europe  for  blasting  in  the 
twelfth  century.  Firearms  do  not  appear  to  have  been  manu- 
factured before  the  close  of  the  fourteenth  century.^  It  is 
probable  that  both  gunpowder  and  the  compass  were  known 
to  the  Chinese  and  the  Hindus  long  before  the  thirteenth  cen- 
tury. 

There  is  no  explicit  evidence  that  the  Chinese  had  any 
knowledge  of  the  magnet  earlier  than  121  a.d.,^  but  there  are 
obscure  passages  in  Chinese  legends  regarding  south-pointing 
chariots  which  have  been  believed  by  some  to  prove  th.at  the 
land  compass  was  used  in  remotest  antiquity.^  No  definite 
testimony  concerning  the  land  compass  occurs  before  the  close 
of  the  eleventh  century.  A  Chinese  author  of  that  time  says 
that  "the  soothsayers  rub  a  needle  with  the  magnet  stone,  so 
that  it  may  mark  the  south ;  however,  it  declines  constantly  a 
little  to  the  east.  It  does  noJLindicate  the  south  exactly."^ 
This  passage  discloses  a  knowledge  of  magnetic  declination. 

As  to  the  mariner's  compass,  an  old  Chinese  encyclopaedia 
says  that  "under  the  Tsin  dynasty  [265  to  419  a.d.]  there 
were  also  ships  indicating  the  south."  This  sentence  is  incon- 
clusive ;  in  fact,  no  truly  reliable  passage  has  been  found  to 

1  RosENBEEGER,  Part  I.,  p.  97. 

2  Klaproth,  Lettre  a  M.  le  Baron  Humboldt  sur  V invention  de  la 
Boussole,  Paris,  1834,  p.  66. 

^  Benjamin,  pp.  63-74.  *  Quoted  by  Benjamin,  p.  76. 


GUNPOWDER    AND   MARINER'S    COMPASS  23 

show  its  use  on  Chinese  waters  prior  to  the  close  of  the 
thirteenth  century. 

There  is  no  good  evidence  to  support  the  claim  that  the 
compass  was  brought  from  China  to  Europe  by  the  Arabs. 
Moreover,  there  is  reason  for  believing  ^'that  the  Orient  re- 
ceived the  better  arrangement  of  the  compass  from  Europe."  ^ 

In  Europe,  the  first  mention  of  the  mariner's  compass  is 
made  in  the  twelfth  century  by  Alexander*  Neckam  of  St. 
Albans,  England.  Another  reference  occurs  in  a  poem  pub- 
lished about  the  close  of  that  century  by  the  Frenchman 
Guyot  de  Provins,  who  speaks  of  the  ugly  brown  stone  to 
which  iron  turns,  through  which  navigators  possess  an  art 
that  cannot  fail  them.  A  bishop  of  Palestine  in  1218  says 
that  the  needle  is  '^  most  necessary  for  such  as  sail  at  sea." 

The  old  mariner's  compass  was  operated  in  a  very  primitive 
manner.  In  a  work  of  1282  an  Arabic  writer  says  that  the 
needle  was  floated  in  a  basin  of  water  by  being  placed  inside 
a  reed  or  upon  a  splinter  of  wood.  When  brought  to  rest  the 
magnet  pointed  north  and  south.  A  similar  practice  seems  to 
have  prevailed  among  the  early  Italians. 

Remarkable  progess  in  the  knowledge  of  magnetism  and  the 
construction  of  the  compass  is  indicated  in  a  letter  written 
August  12,  1269,  by  Master  Peter  de  Maricourt  of  France, 
commonly  called  Peregrinus.  This  man  was  greatly  admired 
by  Eoger  Bacon,  and  for  good  reason.  His  letter  discloses  a 
knowledge  of  magnetic  polarity,  states  that  the  fragments  of  a 
divided  magnet  have  each  two  poles,  gives  the  law  that  unlike 
poles  attract  each  other,  and  mentions  that  a  strong  magnet 
will  reverse  the  polarity  of  a  weaker  magnet.  Peregrinus 
invented  a  compass  with  a  graduated  scale  and  pivoted  needle. 
He  designed  perpetual-motion  machines   based  on  magnetic 

1  A  ScHiJCK  in  Wiedemann's  Beibldtter,  Vol.  17,  1893,  p.  1107. 


24  A   IQSTOKY   OF   PHYSICS 

attraction,  but  was  very  politic,  throwing  the  burden  of  suc- 
cess or  failure  upon  the  makers.  He  himself  was  at  that  time 
a  soldier  and  probably  had  no  tools  for  the  construction  of 
complicated  machines.  His  letter  was  written  from  the 
trenches  in  front  of  Lucera  (a  town  in  southern  Italy,  then 
besieged  by  Charles  of  Anjou).* 

After  Peregrinus  the  graduated  circle  was  replaced  by  the 
"  Rose  of  the  Winds,"  consisting  of  a  star  of,  usually,  thirty- 
two  points.^  In  recent  years  there  has  been  a  tendency  to 
return  to  Peregrinus's  circle,  graduated  in  degrees. 

In  the  Exchange  in  Naples  is  a  brass  statue  erected  to  Flavio 
Oioja  as  the  inventor  of  the  compass  in  1302.  This  man,  a 
resident  of  Amalfi  in  southern  Italy,  has  long  been  considered 
its  originator.  We  know  now  that  it  was  used  in  Europe  before 
his  day,  but  he  probably  identified  himself  with  it  by  introduc- 
ing improvements  in  its  construction. 

An  important  innovation  was  the  suspension  of  the  magnet 
in  gimbal  rings,  known  as  "  Cardan's  suspension."  But  Cardan 
(1501-1576)  does  not  claim  the  invention,  nor  was  it  first 
designed  for  use  with  the  compass.  He  describes  a  chair 
which  had  been  constructed  for  an  emperor,  permitting  that 
royal  personage  to  sit  in  it  during  a  drive  without  expe- 
riencing the  least  jolting.  Cardan  remarks  that  the  same 
arrangement  had  been  used  previously  in  connection  with  oil 
lamps.^ 

1  Peregrinus's  letter  was  printed  in  1558.  It  is  reprinted  in  Hellmann's 
Neudrucke  von  Schriften  u.  Kartell  uber  Meteor,  u.  Erdmagn,  No.  10, 
Berlin,  1898.     See  also  Benjamin,  pp.  165-187. 

2  For  their  various  forms  consult  A.  Breusing,  Die  Nautischen  Instru- 
mente  bis  zur  Erfindung  des  Spicgelsextanten,  Bremen,  1890,  pp.  5-24. 

3  Breusing,  op.  cit.,  p.  16;  Cardan,  De  subtilitate,  Lib.  XVII..,  de 
artibus  artificiosisque  rebus,  Basil,  1560,  p.  1028.  A  remarkable  form  of 
compass  was  patented  in  1876  by  Sir  William  Thomson.  See  article 
" Compass"  in  the  Encyclopaedia  Britannica,  9th  ed. 


HYDEOSTATICS  25 

HYDROSTATICS 

The  application  of  the  principle  of  Archimedes  to  the 
famous  problem  of  the  crown  alleged  by  its  maker  to  be  pure 
gold,  though  really  alloyed  with  silver,  is  explained  in  a  manu- 
script of  the  tenth  century.^  A  treatise,  prepared  perhaps  in 
the  thirteenth  century,  explains  how  to  find  the  volume  of 
irregular  bodies  by  the  method  of  Archimedes,  and  emphasizes 
the  practical  value  of  this  procedure  by  pointing  out  that  the 
prices  of  some  kinds  of  merchandise  depend  upon  size.  In 
this  manuscript  for  the  first  time,  says  Thurot,  occurs  the 
name  "specific  gravity.^^ 

The  Archimedean  principle  and  the  crown  problem  became 
favourite  subjects  with  mathematicians,  but  received  less  atten- 
tion from  philosophers.  As  late  as  1614  KecJcerman,  a  promi- 
nent  student  of  Aristotle,  promulgated  absurdities  like  the 
following:  "Gravity  is  a  motive  quality,  arising  from  cold, 
density,  and  bulk,  by  which  the  elements  are  carried  down- 
wards." "Water  is  the  lower,  intermediate  element,  cold  and 
moist."  2  It  was  taught  by  the  philosophers  that  water  has  no 
gravity  in  or  on  water,  since  it  is  in  its  own  place,  that  air  has 
no  gravity  on  water,  that  water  rises  in  a  pump,  because  nature 
abhors  a  vacuum.^  So  firmly  established  were  these  false 
maxims  regarding  pressure  that  when  Boyle  published  his 
experimental  results  on  the  mechanics  of  fluids,  which  con- 
tradicted Aristotelian  opinions,  he  felt  constrained  to  advance 
his  views  under  the  title  of  "hydrostatic  paradoxes."  ^ 

1  Ch.  Thurot,  Principe  d''Archimede,  p.  27. 

2  Whewell,  Vol.  I.,  1858,  p.  236.  3  Jticlem,  p.  236. 
*  Ibidem,  pp.  189,  236. 


26  A    HISTORY    OF   PHYSICS 

LIGHT 

In  the  thirteenth  century  Europe  was  assimilating  the  sci- 
ence of  optics  as  obtained  from  the  Arabs.  Wilhelm  von  Moer- 
beck,  in  1278  Archbishop  of  Corinth,  translated  into  Latin 
Al  Hazen's  treatise  on  parabolic  mirrors.  About  1270  his  friend 
Witelo,  or  Vitellio,  a  Thuringian  monk,  prepared  a  work  on 
optics  less  diffuse  and  more  systematic  than  that  of  Al  Hazen, 
on  which  it  was  based.  Witelo  explained  the  twinkling  of 
stars  as  due  to  the  motion  of  the  air,  and  showed  that  the 
effect  was  intensified,  if  the  star  was  viewed  through  water 
in  motion.  He  pointed  out  that  the  rainbow  was  not  formed 
by  reflection  alone,  as  was  taught  by  Aristotle,  but  was  due 
to  both  reflection  and  refraction. 

Prominent  among  writers  of  the  Middle  Ages  who  drew 
from  Arabic  sources  was  Roger  Bacon  (1214  ?-1294).  He 
wrote  on  optics,  and  by  mistake  has  been  credited  with  the 
invention  of  the  refracting  telescope.  No  doubt.  Bacon  sus- 
pected the  possibility  of  designing  an  instrument  which  should 
enable  one  to  "read  the  smallest  writing  at  enormous  dis- 
tances "  from  the  eye.  But  Bacon  never  constructed,  or  tried 
to  construct,  such  an  instrument.  The  claim  set  up  for  him 
grew  out  of  a  mistranslation  of  a  passage  in  his  works.^ 

Bacon  was  one  of  the  most  gifted  minds  of  the  Middle 
Ages.  Educated  at  Oxford  and  Paris,  he  became  famous  as 
professor  at  Oxford.  His  open  contempt  for  scholasticism  and 
for  immorality  among  the  clergy  led  to  the  charge  of  heresy 
and  to  imprisonment.  From  his  Oxford  cell  he  sent  out  an 
appeal  for  experimental  science  which  nearly  converted  his  old 
friend  Pope  Clement  IV.  But  Bacon's  ideas  were  in  advance 
of  his  time  and  bore  no  immediate  fruit.  In  Paris  he  was 
imprisoned  a  second  time  for  a  period  of  ten  years.  Thus 
the  genius  of  this  remarkable  man  was  crushed  by  the  politi- 
cal and  mental  despotism  of  his  time. 

1  E.  Wiedemann  in   Wiedemann'' s  Annalen,  Vol.  39,  1890,  p.  130. 


THE  RENAISSANCE 

The  sixteenth  century  was  a  period  of  intense  intellectual 
activity.  The  minds  of  men  were  cut  adrift  from  their  ancient 
moorings  and  launched  forth  on  the  wide  sea  of  inquiry. 

The  movement  was  of  great  breadth.  Hei-e  we  witness  the 
revival  of  classic  learning,  there  the  production  of  masterpieces 
in  art  by  Michel  Angelo,  Raphael,  and  Da  Vinci.  Yonder 
we  behold  the  stupendous  struggle  against  Church  authority, 
known  as  the  Reformation.  The  secluded  mathematician  in- 
fuses new  life  into  algebra  and  trigonometry.  The  astronomer 
gazes  at  the  stars  and  creates  a  new  system  of  the  universe. 
The  physicist  abandons  scholastic  speculation  and  begins  to 
study  nature  in  the  language  of  experiment. 


THE   COPEENICAN   SYSTEM 

The  first  great  scientific  victory  during  the  Renaissance  was 
the  overthrow  of  the  Ptolemaic  and  the  establishment  of  the 
Copernican  System.  We  shall  pause  a  moment  to  consider 
briefly  this  great  epoch  in  the  development  of  our  sister 
science,  astronomy. 

The  Greek  astronomers,  Eudoxus  and  Hipparchus,  explained 
planetary  motions  by  the  famous  theory  of  epicycles  and  eccen- 
trics. The  apparent  sweep  of  the  planet  around  the  earth  was 
represented  by  the  combination  of  two  motions  :  (1)  the  yearly 
motion  of  the  planet  along  the  circumference  of  a  small  circle, 
called  the   epicycle;    (2)   the   motion   of   the   centre   of   that 

27 


28  A   HISTORY   OF   PHYSICS 

epicycle  along  the  circumference  of  a  second  circle  which, 
surrounds  the  earth.  We  know  now  that  the  latter  circle 
represents  approximately  the  true  orbit  of  the  planet  around 
the  sun  and  that  the  epicyclic  motion  is  only  apparent.  This 
apparent  motion  is  due  to  the  real  motion  of  the  earth  itself. 
If  an  observer  is  carried  around  in  a  circle,  then  an  object  at 
rest  will  appear  to  him  to  move  in  a  circle  of  equal  size.  The 
ancient  theory  is,  therefore,  approxima.tely  correct,  the  main 
error  lies  in  attributing  to  the  planets  oscillations  which  do 
not  really  exist,  but  which  the  planets  seem  to  have  on  account 
of  the  orbital  motion  of  the  earth.  Hipparchus  observed  that 
the  theory  of  epicycles  could  not  explain  the  motions  of  the 
planets,  if  the  earth  must  be  assumed  to  be  exactly  in  the 
centre  of  the  second  circle  mentioned  above.  This  led  him 
to  establish  the  theory  of  the  eccentric. 

This  ancient  system  was  elaborated  by  and  named  after  the 
distinguished  Alexandrian  astronomer  Claudius  Ptolemy.  It 
made  the  earth  immovable  at  the  centre  of  the  universe. 
Around  it  revolved  in  successively  wider  spheres  the  Moon, 
Mercury,  Venus,  Sun,  Mars,  Jupiter,  Saturn,  and  lastly  the 
eighth  sphere  of  the  fixed  stars. 

This  geocentric  theory  of  the  universe  has  always  had  its 
opponents,  but  it  was  first  vigorously  attacked  by  Nicolaus 
Copernicus  (1473-1543).  He  was  probably  of  Polish  descent 
and  was  born  at  Thorn,  in  Prussia,  near  the  Polish  boundary. 
For  twenty-three  years  he  was  engaged  in  the  threefold  occu- 
pation of  discharging  ecclesiastical  duties,  practising  medicine, 
and  studying  astronomy.  With  the  hope  of  finding  an  expla- 
nation of  less  complexity  than  that  offered  by  the  Ptolemaic 
system,  he  zealously  studied  all  sources  of  information  at  his 
command.  He  found  that  all  sorts  of  opinions  had  been 
advanced.  Thus,  the  Pythagoreans  believed  in  the  rotation 
of  the  earth,  and  Philolaus  had  even  imagined  the  earth  to 


THE   COPERNICAN   SYSTEM  29 

have  an  orbit  around  tlie  sun.  Aided  by  suggestions  of  this 
sort,  Copernicus  gradually  matured  his  own  system.  For 
many  years  he  withheld  from  publication  the  manuscript  of 
his  De  orbium  codestium  revolutionibus,  but  finally,  in  1542, 
he  consented  to  have  it  printed.  He  died  before  the  printing 
was  completed.  This  saved  him  from  persecution.  Others 
—  Giordano  Bruno  and  Galileo  —  had  to  suffer  for  the  Coper- 
nican  system. 

Copernicus  taught  that  the  earth  was  spherical,  rotated 
on  its  axis  and  revolved  around  the  sun ;  that  the  motions  of 
the  heavenly  bodies  are  either  circular  and  uniform  or  com- 
pounded of  circular  and  uniform  motions.  He  explained  for 
the  first  time  the  variation  of  the  seasons  and  the  cause  of 
the  apparent  oscillations  of  the  planets.  A  great  defect  in 
his  system  was  his  notion  that  all  celestial  motions  are  com- 
pounded of  circular  ones.  It  cannot  be  said  that  the  argu- 
ment made  by  Copernicus  against  the  Ptolemaic  system  was 
conclusive.  To  overthrow  completely  the  ancient  theory 
required  the  genius  of  another  man  —  Kepler. 

Johannes  Kepler  (1571-1630)  was  at  one  time  in  Prague 
assistant  to  the  Danish  astronomer  Tycho  Brake.  Unlike 
Tycho,  Kepler  had  no  talent  for  observation  and  experimenta- 
tion. But  he  was  a  great  thinker  and  excelled  as  a  mathe- 
matician. He  absorbed  Copernican  ideas,  and  early  grappled 
with  the  problem  of  determining  the  real  paths  of  the  planets. 
In  his  first  attempts  he  worked  on  the  dreams  of  the  Pythag- 
oreans concerning  figure  and  number.  Intercourse  with 
Tycho  led  him  to  reject  such  mysticism  and  to  study  the 
observations  on  the  planets  recorded  by  his  master.  He  took 
the  planet  Mars,  and  found  that  no  combinations  of  circles 
would  give  a  path  which  could  be  reconciled  with  the  actual 
observations.  In  one  case  the  difference  between  the  observed 
and  his  computed  values  was  eight  minutes,  and  he  knew  that 


30  A   HISTORY   OF   PHYSICS 

SO  accurate  an  observer  as  Tycho  could  not  make  an  error  so 
great.  He  tried  an  oval  orbit  for  Mars,  and  rejected  it ;  he 
tried  an  ellipse,  and  it  fitted!  Thus,  after  more  than  four 
years  of  assiduous  computation,  and  after  trying  nineteen 
imaginary  paths  and  rejecting  each  because  it  was  more  or 
less  inconsistent  with  observation,  Kepler  in  1618  discovered 
the  truth.  An  ellipse !  Why  did  he  not  think  of  it  before  ? 
What  a  simple  matter  —  after  the  puzzle  is  once  solved.  He 
worked  out  what  are  known  as  "Kepler's  Laws,"  which  ac- 
corded with  observation  but  conflicted  with  the  Ptolemaic 
hypothesis.  Thus  the  old  system  was  logically  overthrown. 
But  not  until  after  a  bitter  struggle  between  science  and 
theology  did  the  new  system  find  general  acceptation.^ 

MECHANICS 

The  sixteenth  century  witnessed  the  revival  of  statics  and 
the  creation  of  dynamics.  The  science  of  statics,  which,  since 
the  time  of  Archimedes,  had  been  nearly  stationary,  was  first 
taken  up  by  Simon  Stevin  (1548-1620)  of  Bruges  in  Belgium, 
a  man  remarkable  for  varied  attainments  in  science,  indepen- 
dence of  thought,  and  extreme  lack  of  respect  for  authority. 
He  is  the  inventor  of  decimal  fractions.  In  1605  he  published 
at  Leyden  a  work  written  in  Dutch,  which  in  1608  was  brought 
out  in  Latin  translation  under  the  title  Hypomnemata  mathe- 
matica.  Stevin  accurately  determined  the  force  necessary  to 
sustain  a  body  on  an  inclined  plane  and  investigated  the  equi- 
librium of  pulleys.  He  employed  the  principle  of  the  paral- 
lelogram of  forces,  but  did  not  expressly  formulate  it.  In 
fact,  he  was  in  possession  of  a  complete  doctrine  of  equi- 

1  For  an  account  of  this  struggle,  consult  A.  D.  White,  The  Warfare 
of  Science  with  Theology,  New  York,  1896,  Vol.  I.,  pp.  114-170. 


MECHANICS  31 

librium.^     Da  Vinci^  the  famous  pamter,  Guido  Ubaldi,  and 
Galileo  paid  some  attention  to  statics. 

The  creation  of  the  science  of  dynamics  is  due  to  Galileo 
Galilei  (1564-1642),  a  native  of  Pisa.  He  studied  medicine  at 
the  University  of  Pisa,  but  in  a  few  years  he  abandoned  it 
for  the  more  congenial  pursuit  of  mathematics  and  science. 
In  1589  he  received  the  appointment,  for  three  years,  to  the 
mathematical  chair  in  Pisa.  During  this  time  he  performed 
memorable  experiments  on  falling  bodies,  but  his  new  views 
met  with  so  much  opposition  that  he  was  obliged  to  resign  in 
1591.  From  1592  to  1610  he  was  professor  at  Padua.  There- 
upon he  began  boldly  to  preach  Copernican  doctrines.  In 
consequence  he  was  summoned  before  the  Inquisition  at 
Rome.  The  theory  of  the  earth's  motion  was  condemned  by 
the  Inquisition,  and  Galileo  received  an  injunction  to  silence. 
For  some  years  Galileo  remained  silent,  though  always  at 
work.  In  1632  he  published,  contrary  to  the  edict  of  1616,  a 
new  work,  the  Dialogo,  which  was  a  brilliant  success  as  an 
argument  in  favour  of  the  Copernican  theory.  This  brought 
about  a  second  trial.  The  old  man  of  seventy  was  subjected 
to  indignity,  imprisonment,  and  threats.  On  his  knees  he  was 
forced  publicly  to  "  abjure,  curse,  and  detest  the  error  and  the 
heresy  of  the  movement  of  the  earth."  ^  At  first  he  was  kept 
in  separation  from  his  family  and  friends,  but  was  allowed 

1  For  details  consult  E.  Mach,  Science  of  Mechanics  (ed.  McCormack), 
pp.  24-34. 

2  Quoted  by  A.  D.  White,  op.  cit.^  Vol.  I.,  p.  142.  After  the  abjura- 
tion, as  Galileo  arose  from  his  kneeling  posture,  he  is  said  to  have  mur- 
mured the  words,  "  Eppur  si  muove  "  ("and  yet  it  moves").  Upon  a 
careful  study  of  documents,  G.  Berthold  comes  to  the  conclusion  that 
this  story  is  legendary.  Yet  there  can  be  no  doubt  that  the  "Eppur  si 
muove"  expresses  what  must  have  been  Galileo's  innermost  conviction. 
See  Berthold,  Zeitschr.  f.  Math,  und  Phys.,  Vol.  42,  1897,  pp.  5-9  i 
K.  Wolf,  Gesch.  d.  Asti'onomie,  Mlinchen,  1877,  p.  262. 


32  A  HISTORY   OF  PHYSICS 

a  little  more  liberty  after  he  became  blind  and  wasted  with 
disease.^ 

Galileo  was  among  the  first  to  teach  that  the  Holy  Script- 
ures were  not  intended  as  a  text-book  on  science  —  a  truth 
which  the  world  has  been  slow  to  recognize. 

The  first  years  after  1632  were  given  to  the  study  of  dy- 
namics. In  1638  appeared  in  Leyden  his  dialogues  on  motion, 
under  the  title,  Discorsi  e  dimostrazioni  matematiche.  These 
now  are-  considered  his  greatest  and  most  substantial  achieve- 
ment. 

The  first  experiments,  which  Galileo  made  while  he  was  a 
young  professor  at  Pisa,  were  decidedly  dramatic.  At  that 
time  the  doctrine  that  the  rate  at  which  a  body  falls  depends 
upon  its  weight  was  generally  accepted  as  true,  merely  on  the 
authority  of  Aristotle.  It  was  even  held  that  the  acceleration 
varies  as  the  weight.  Prior  to  Galileo  it  did  not  occur  to 
any  one  actually^to,  try  the  experiment.  The  young  profes- 
sor's tests  went  contrary  to  the  doctrine  held  for  two  thousand 
years.  Allowing  for  the  resistance  of  the  air,  he  found  that 
all  bodiesfe]I_jjt_jlie_^sg^^  the  distance  passed 

overva£ied_3sJh^,jgU^re,.ofth^  time.  With  all  the  enthusi- 
asm, courage,  and  imprudenceof^outh,  the  experimenter  pro- 
claimed that  Aristotle,  at  that  time  believed  by  nearly  every 
one  to  be  verbally  inspired,  was  wrong.  Galileo  met  with  op- 
position, but  he  decided  to  give  his  opponents  ocular  proof. 
It  seems  almost  as  if  nature  had  resorted  to  an  extraordinary 
freak  to  furnish  Galileo  at  this  critical  moment  in  the  history 
of  science,  with  an  unusual  convenience  for  his  public  demon- 
stration. Yonder  tower  of  Pisa  had  bent  over  to  facilitate  ex- 
perimentation, from  its  top,  on  falling  bodies.  One  morning, 
before   the   assembled  university,   he   ascended    the    leaning 

1  A.  D.  White,  op.  cit.,  pp.  142,  143. 


MECHANICS  33 

cower,  and  allowed  a  one  pound  shot  and  a  one  hundred  pound 
shot  to  drop  together.  The  multitude  saw  the  balls  start 
together,  fall  together,  and  heard  them  strike  the  ground 
together.  Some  were  convinced,  others  returned  to  their 
rooms,  consulted  Aristotle,  and,  distrusting  the  evidence  of 
their  senses,  declared  continued  allegiance  to  his  doctrine. 

The  crooked  path  by  which  discoveries  are  sometimes  made 
is  curiously  illustrated  in  the  assumption  at  first  made  by 
Galileo  regarding  the  nature  of  uniformly  accelerated  motion. 
He  takes  the  velocity  to  be  proportional  to  the  distance  passed 
over,  and  then,  by  a  train  of  reasoning  which  we  find  itself  to 
be  fallacious,  concludes  that  this  assumption  is  erroneous. 
^'  If  the  velocity  with  which  a  body  overcomes  four  yards  is 
double  the  velocity  with  which  it  passed  over  the  first  two 
yardSj  then  the  times  necessary  for  these  processes  must  be 
equal ;  but  four  yards  can  be  overcome  in  the  same  time  as 
two  yards  only  if  there  is  an  instantaneous  motion.  We  see, 
however,  that  the  body  takes  time  in  falling  and  requires, 
indeed,  less  time  for  a  fall  of  two  than  of  four  yards.  Hence 
it  is  not  true  that  the  velocity  increases  proportionally  to  the 
distance  fallen."  ^ 

Galileo  then  proceeds  to  a  second  assumption,  —  velocity  is 
jgropQrtional  to  the  time  of  falling,  —  and,  finding  no  self- 
contradiction  in  it,  he  goesabout  to  test  it  experimentally. 
In  a  board  twelve  yards  long  a  trough  one  inch  wide  was  cut 
out  in  a  straight  line  and  lined  with  very  smooth  parchment. 
A  brass  ball,  perfectly  round  and  polished,  was  allowed  to 
run  down  the  inclined  plane.     About  one  hundred  trials  were 

1  Galileo's  Discorsi  of  1638  were  in  1890  published  in  German  transla- 
tion in  OstwaWs  Klassiker  der  exacten  Wissenschciften,  Nos.  11,  24,  25. 
The  above  quotation  is  from  No.  24,  p.  17.  A  new  and  complete  edition 
of  Galileo's  works  has  been  prepared  recently  for  the  Italian  government 
by  Antonio  Favaro,  of  Padua. 


34 


A   HISTORY    OF   PHYSICS 


1 


E     F     B 
Fig.  3 


made  for  different  inclinations  and  lengths  of  the  plane.     The 
distance  of  descent  was  found  always  to  vary  closely  as  the 
squares  of  the  times.     It  is  interesting  to  notice  how  Galileo 
C    measured  the  time.     Accurate  clocks  or  watches 
were   then  not   available.     He  attached  a  very 
small   spout  to  the  bottom  of  a  water  pail  and 
caught  in  a  cup  the  water  escaping  through  the 
spout  during  the  time  when  the  body  travelled 
through  a  given  distance.    The  water  was  weighed 
accurately  and  the  times  of  descent  taken  pro- 
portional to  the  ascertained  weight.^ 

To  exhibit  the  relation  between  velocity  and 
distance,  Galileo  establishes  the  theorem  that 
the  time  in  which  a  body  moving  from  rest  with 
uniformly  accelerated  velocity  travels  a  given  distance  is  the 
same  as  the  time  it  would  require  to  travel  the  same  distance 
with  a  uniform  velocity  equal  to  half  its  actual  final  velocity. 
This  truth  he  illustrated  by 
Fig.  3.2  The  line  EB  rep- 
resents the  final  velocity, 
which  varies  directly  as  the 
time  represented  by  AB. 
The  area  ABE  stands  for 
the  distance  gone  over.  This 
area  is  evidently  equal  to 
that  of  the  rectangle  ABFG, 
where  FB  stands  for  the 
average  velocity.  This  geometrical  illustration  has  retained 
a  place  in  some  modern  text-books.  Still  more  common  is  the 
illustration  (Fig.  4)  showing  the  path  of  a  body  projected 
horizontally  and  acted  upon  by  gravity.^     In  the  dialogue  on 

1  Ostwald's  Klassiker^  No.  24.  p.  2-5. 

2  Ibidem,  p.  21.  ^  Ji^idem,  p.  84. 


E            1 

D           C           B 

J. 

^-- — 

0 

F. 

y^ 

G 

R. 

L 

N 

Fig.  4. 


MECHANICS  35 

this  subject  Galileo  permits  Sagredo  to  remark  naively,  "  Truly 
the  conception  is  new,  ingenious,  and  incisive ;  it  rests  on  an 
issumption,  namely,  that  the  transverse  motion  remains  con- 
stant and  that,  at  the  same  time,  the  naturally  accelerated 
motion  maintains  itself,  proportional  to  the  squares  of  the 
times,  and  that  such  motions  mix,  indeed,  but  do  not  disturb, 
change,  and  impede  each  other,  so  that  finally,  by  the  pro- 
gressive motion,  the  path  of  the  projectile  is  not  degenerated 
—  a  behaviour  hardly  comprehensible  to  me." 

Galileo  was  the  first  to  show  that  the  path  of  a  projectile 
is  a  parabola.  Previously  it  wa^^elieved  by  some  that  a 
cannon  ball  moved  forward  at  first  in.-ar-sti:aigktjine_aiid_-then 
suddenly  fell  vertically  tojihfi_ground. 

Galileo  had  an  understanding  of  centrifugal  force  and  gave 
a  correct  definition  of  momentum.  With  Stevin  and  others  he 
also  wrote  on  statics.  He  formulated  the  principle  of  the 
parallelogram  of  forces,  but  he  did  not  fully  recognize  its  scope. 

Still  another  subject  engaging  Galileo's  attention  was  the 
laws  of  the  pendulum.  As  in  case  of  falling  bodies,  so  here 
the  first  observations  were  made  while  he  was  a  young  man. 
In  1583,  while  he  was  praying  in  the  cathedral  at  Pisa,  his 
attention  was  arrested  by  the  motion  of  the  great  lamp  which 
after  being  lighted  had  been  left  swinging.  Galileo  proceeded 
to  time  its  oscillations  by  the  only  watch  in  his  possession, 
namely,  his  own  pulse.  He  found  the  times,  as  near  as  he 
could  tell,  to  remain  the  same,  even  after  the  motion  had 
greatly  diminished.  Thus  was  discovered  the  isochronism  of 
the  pendulum.  Galileo  was  at  that  time  studying  medicine, 
and  he  applied  the  pendulum  to  pulse  measurements  at  the 
sick-bed.  He  also  proposed  its  use  in  astronomical  observa- 
tions. More  careful  experiments  carried  out  by  him  later,  and 
described  in  his  Discorsi,  showed  that  the  time  of  oscillation 
was  independent  of  the  mass  and  material  of  the  pendulum 


36  A   HISTORY   OF   PHYSICS 

and  varied  as  the  square  root  of  its  length.^  His  last  contri« 
biition  to  the  art  of  time  measurement  was  made  after  he  had 
become  blind.  In  1641  he  dictated  to  his  son  Vicenzo  and  his 
pupil  Viviani  the  description  and  drawing  of  a  pendulum 
clock.  The  original  drawing  is  extant,  but  a  model,  said  to 
have  been  constructed  by  Yiviani  in  1649,  has  been  lost. 
Galileo's  invention  did  not  become  generally  known  at  that 
time,  and  fifteen  years  later,  in  1656,  Christian  Huygens  inde- 
pendently invented  a  pendulum  clock,  which  met  with  general 
and  rapid  appreciation.  The  honour  of  this  great  invention 
belongs,  therefore,  to  Galileo  and  Huygens.^ 

Galileo's  Discorsi  of  1638  are  masterpieces  of  popular  expo- 
sition, which  fact  alone  renders  them  worthy  of  perusal.  But 
they  contain  other  points  of  merit.  W.  G.  Adams  well  says : 
"The  true  method  of  teaching  mechanics  is  illustrated  by  the 
way  in  Avhich  Galileo  established  the  first  principles  of 
dynamics,  and  placed  them  before  his  pupils.  Due  weight 
should  be  given  both  to  experimental  and  to  rational  mechanics, 
and  the  best  way  of  bringing  the  subject  before  students  is  to 
have  parallel  but  distinct  courses  of  experimental  and  theo- 
retical lectures  attended  by  students  at  the  same  time."^ 

Among  his  contemporaries  it  was  chiefly  the  novelties  he 
detected  in  the  skies  that  made  him  celebrated,  but  Lagrange 


1  Ostwald''s  Klassiker,  No.  11,  pp.  75,  84. 

2  The  invention  of  the  pendulum  clock  has  been  claimed  also  for  the 
Swiss  Joost  Biirgi  (R.  Wolf,  Geschichte  der  Astronomie,  1877,  p.  369), 
for  Richard  Harris  of  London  (Edinburgh  Encyclopcedia,  1830,  Vol.  11, 
p.  117),  and  for  others,  but  these  claims  have  been  rejected  by  later 
authorities.  On  the  history  of  this  invention  consult  E.  Gerland, 
Zeitschr.  f.  Instrumenten  Kunde,  Vol.  VIII.,  1888,  p.  77:  W.  C.  L.  v. 
ScHAiK  in  same  journal.  Vol.  VII.,  pp.  350  and  428;  S.  Gijnther, 
Vermischte  Untersuchung en,  Leipzig,  1876,  pp.  308-344;  G.  Bekthold, 
Schlomilch's  Zeitschr.,  Vol.  38, 1893,  Hist.  Lit.  Abth.,  p.  123. 

3  Nature,  Vol.  V.,  1871-1872,  p.  389. 


LIGHT  37 

claims  that  his  astronomical  discoveries  required  only  a  tele- 
scope and  perseverance,  while,  in  the  case  of  dynamics,  it  took 
an  extraordinary  genius  to  discover  laws  from  phenomena 
which  we  see  constantly  and  of  which  the  true  explanation 
escaped  all  earlier  philosophers. 

LIGHT 

The  greatest  achievement  in  optics  during  the  Renaissance 
was  the  invention  of  instruments,  giving  an  observer  a  glimpse 
of  the  infinitely  distant  and  of  the  infinitely  small.  We  refer 
to  the  telescope  and  the  microscope. 

According  to  tradition  the  telescope  was  invented  by  acci- 
dent. The  great  Huygens  in  his  Dioptrica  asserts  that  a  man 
capable  of  inventing  the  telescope  by  mere  thinking  and 
application  of  geometrical  principles,  without  the  concurrence 
of  accident,  would  have  been  gifted  with  superhuman  genius. 
To  this  remarkable  statement  Mach  adds  that  it  does  not  follow 
that  accident  alone  is  sufiicient  to  produce  an  invention.  The 
inventor  "must  distinguish  the  new  feature,  impress  it  upon 
his  memory,  unite  and  interweave  it  with  the  rest  of  his 
thought;  in  short,  he  must  possess  the  capacity  to  projit  by 
experience.^^  ^ 

There  have  been  brought  forward  numerous  candidates  for 
the  honor  of  the  invention  of  these  marvellous  instruments. 
Four  nations,  the  English,  Italian,  Dutch,  and  German,  have 
each  endeavoured  to  secure  a  decision  in  favour  of  one  of  its 
own  countrymen. 

The  evidence  we  possess  favours  the  Dutch.  The  first  tele- 
scope was  probably  constructed  in  1608  by  Hans  Lii^j^erslieyj 
a  native  of  Wesel,  and  a  manufacturer  of  spectacles  in  Mid- 

1  E.  Mach,  "  On  the  Part  Played  by  Accident  in  Invention  and  Dis- 
covery," Monist,  Vol.  VI.,  p.  166. 


38  A   HISTORY   OF   PHYSICS 

dleburg.^  He  prepared  his  lenses,  not  of  glass,  but  of  rock 
crystal.  A  document  found  in  the  archives  at  the  Hague  shows 
that  on  October  2,  1608,  he  applied  for  a  patent.  He  was 
told  to  modify  his  construction  and  make  an  instrument  en- 
abling the  observer  to  see  through  it  with  both  eyes.  This  he 
accomplished  the  same  year.  He  did  not  receive  his  patent, 
but  the  government  of  the  United  Netherlands  paid  him  in- 
stead 900  gulden  for  the  instrument  and  an  equal  sum  for  two 
other  binocular  telescopes,  completed  in  1609.^ 

The  invention  of  the  microscope  is  nearly  contemporaneous 
with  that  of  the  telescope.  It  is  now  usually  ascribed  to 
Zacliarias  Joannides  and  his  father,  though  Huygens  assigned 
it  to  Cornelius  DrehheV    At  first  the  eye-pieces  consisted  of 

1  Dr.  H,  Servus,  Die  Geschichte  des  Fernrohrs,  Berlin,  1886,  p.  39. 

2  Ibidem,  p.  40.  The  claim  that  Eager  Bacon  invented  the  telescope 
is  now  generally  abandoned.  The  Italian  Giambattista  della  Porta, 
known  as  the  inventor  of  the  camera  obscura,  has  been  named  in  this 
connection  on  the  strength  of  passages  in  his  Magia  Naturalis  (2d  Ed., 
1589)  to  the  effect  that  by  judicious  combination  of  two  lenses,  one  con- 
vex and  the  other  concave,  objects  at  a  distance  as  well  as  objects  near  at 
hand  may  be  magnified  to  the  eye.  But  his  experiments  appear  to  have 
been  confined  to  the  preparation  of  suitable  eye-glasses  for  persons  with 
abnormal  vision ;  the  invention  of  the  telescope  is  here  out  of  the  ques- 
tion. In  1571  Leonard  Digges  of  Bristol  published  a  book  in  which  the 
effect  of  combining  concave  and  convex  lenses  is  explained,  somewhat  as 
in  Porta's  book  of  1589,  but  all  statements  of  this  sort  must  be  regarded 
as  having  prepared  for  the  invention  rather  than  as  having  actually  con- 
stituted it.  Previous  to  1831  the  best  evidence  at  hand  seemed  to  point 
to  Zacharias  Joannides  of  Middleburg  in  Netherlands  as  the  inventor 
of  the  telescope,  though  his  countrymen,  Adrian  3Ietius  and  Cornelius 
Drebbel,  the  Germans  Simon  Marius  and  Kepler,  and  the  Italians  Fran- 
ciscus  Fontana  and  Galileo  have  all  had  their  supporters.  All  of  these, 
except  Kepler,  were  actually  engaged  in  the  manufacture  of  telescopes. 

"  G.  Govi  claims  the  invention  of  the  microscope  for  Galileo.  From  a 
document  printed  in  1610  he  proves  that  Galileo  had  modified  the  telescope 
to  see  very  small  and  very  near  objects.  Consult  G.  Govi  in  Bendic. 
Accad.  Napol.,  (2)  I.,  1887  ;  C.  R.  107,  No.  14,  1888;  Poske's  Zeitschr., 


LIGHT  39 

concave  lenses.  Franciscus  Fontana  of  !N'aples  appears  to 
have  been  the  first  to  replace  the  concave  eye-lens  by  a  con- 
vex one.  Kepler  was  the  first  to  suggest  a  similar  change  in 
the  telescope.  All  the  artisans  whom  we  have  mentioned  in 
connection  with  the  microscope  are  known  to  have  been  promi- 
nent in  the  manufacture  of  telescopes. 

The  use  of  the  new  instruments  spread  over  Europe  with 
rapidity.  In  England  the  mathematician  TJiomas  Harriot 
had  a  telescope  magnifying  fifty  times,  and  he  observed  the 
satellites  of  Jupiter  in  1610,  almost  as  early  as  did 
Galileo.! 

The  news  of  the  invention  of  the  telescope  incited  Kepler, 
who  had  already  given  much  time  to  the  study  of  optics,  to 
fresh  efforts.  In  1611  he  published  his  Dioptrice,  which  is 
the  earliest  work  containing  an  attempt  to  elaborate  the  theory 
of  the  telescope.  Such  an  attempt  demands  a  knowledge  of 
the  law  of  refraction.  Kepler  arrived  at  an  empirical  expres- 
sion which  was  merely  an  approximation.  The  accurate  law 
he  failed  to  discover.  His  approximate  result  for  small  angles 
(i  <  30°)  was  I  =  nr,  where  n  is  a  constant,  equal  to  f  for  a  ray 
passing  from  air  to  glass.     This  was  near  enough  to  the  truth 

Zweiter  Jahrgang,  1888,  p.  93.  Galileo  says  in  his  Sidereus  Nimcius^ 
which  was  published  at  the  beginning  of  the  year  1610,  that  he  first  heard 
of  the  invention  of  the  telescope  "about  ten  months  ago."  His  micro- 
scope was  a  modified  telescope.  Hence  his  microscope  must  have  been 
made  in  1609  or  1610.  Now,  if  we  may  trust  the  testimony  contained  in 
a  letter  by  the  Dutch  ambassador,  Borelius,  written  in  1655,  then  Zacharias 
Joannides  did  not  construct  a  telescope  until  1610,  "long  after"  (longe 
post)  he  had  invented  the  microscope.  See  H.  Sekvus,  op.  cit.,  pp.  17, 
18.    According  to  this  Joannides  anticipated  Galileo, 

1  In  1585  Sir  Walter  Raleigh  sent  Harriot  to  Virginia  as  surveyor 
with  Sir  Richard  Grenville's  expedition.  Among  the  mathematical  in- 
struments by  which  the  wonder  of  the  Indians  was  aroused,  Harriot  men- 
tions "a  perspective  glass  whereby  was  showed  many  strange  sights." 
See  Die.  of  Nat.  Biography. 


40  A   HISTORY   OF  PHYSICS 

to  enable  him  to  give  in  broad  outline  the  correct  theory  of  th6 
telescope. 

The  earliest  important  scientific  discoveries  with  the  aid  of 
the  telescope  were  made  by  Galileo.  He  was  led  to  take  up 
this  line  of  research  by  rumours  which  had  reached  him  regard- 
ing the  invention  in  Belgium  of  an  instrument  through  which 
distant  objects  could  be  seen  distinctly.  He  probably  heard 
that  this  had  been  effected  by  the  combination  of  a  concave 
and  a  convex  lens,  and  he  set  to  work  to  devise  such  an  instru- 
ment himself.  Guided  by  the  hints  he  had  received  and  by 
his  knowledge  of  dioptrics,  he  soon  succeeded.  He  made  a 
rough  telescope  with  two  glasses  fixed  at  the  end  of  a  leaden 
tube,  both  having  one  side  flat ;  the  other  side  of  the  one  lens 
being  concave,  and  of  the  other  lens  convex.  It  made  objects 
appear  three  times  nearer  and  nine  times  larger.  Thereupon, 
sparing  neither  expense  nor  labour,  he  got  so  far  as  to  construct 
an  instrument  which  magnified  an  object  nearly  a  thousand 
times  and  brought  it  more  than  thirty  times  nearer.^ 

Galileo  went  to  Venice  and  showed  it  to  the  signoria.  Says 
he:  "Many  noblemen  and  senators,  although  of  great  age, 
mounted  the  steps  of  the  highest  church  towers  at  Venice  to 
watch  the  ships,  which  were  visible  through  my  glass  two 
hours  before  they  were  seen  entering  the  harbour." 

Galileo's  telescopes  were  much  sought  after,  and  he  received 
numerous  orders  from  learned  men,  princes,  and  governments 
—  Holland,  the  birthplace  of  the  telescope,  not  excepted.^ 

Galileo  turned  his  telescope  toward  the  moon  and  discovered 
mountains  and  craters ;  he  turned  it  to  Jupiter  and  saw  its 
satellites  (January  7,  1610) ;  he  pointed  it  at  Saturn  and  saw 

1  Consult  Sidereus  Nuncius  of  1610,  reprinted  in  editions  of  Galileo's 
works ;  also  Karl  ton  Gebler,  Galileo  Galilei  and  the  Moman  Curia, 
trans,  by  Mrs.  George  Sturge,  London,  1879,  p.  17. 

2  Gebler,  op.  cit.,  p.  18. 


ELECTRICITY   AND   MAGNETISM  41 

the  planet  threefold  —  now  known  to  have  been  due  to  an  im- 
perfect view  of  the  ring;  he  examined  the  sun,  saw  its  spots 
moving,  and  concluded  that  the  sun  rotates.  All  this  was 
achieved  in  1610.  His  observations  seemed  to  confirm  the 
Copernican  theory.  The  cloud  of  opposition  to  Galileo  began 
to  gather.  Some  refused  to  believe  their  eyes,  and  asserted 
that,  while  the  telescope  answered  well  enough  for  terrestrial 
objects,  it  was  false  and  illusory  when  pointed  at  celestial 
bodies.  Others  refused  to  look  through  it.  Among  the  latter 
was  a  university  professor.  Galileo  Avrote  to  Kepler:  "Oh, 
my  dear  Kepler,  how  I  wish  that  we  could  have  one  hearty 
laugh  together  !  Here,  at  Padua,  is  the  principal  professor  of 
philosophy,  whom  I  have  repeatedly  and  urgently  requested 
to  look  at  the  moon  and  planets  through  my  glass,  which  he 
pertinaciously  refuses  to  do.  Why  are  you  not  here  ?  What 
shouts  of  laughter  we  should  have  at  this  glorious  folly !  And 
to  hear  the  professor  of  philosophy  at  Pisa  labouring  before  the 
Grand  Duke  with  logical  arguments,  as  if  with  magical  incan- 
tations to  charm  the  new  planets  out  of  the  sky."  ^  The 
antagonism  to  Galileo  and  his  hated  telescope  became  stronger. 
The  clergy  began  to  denounce  him  and  his  methods.  Father 
Caccini  became  known  as  a  punster  by  preaching  a  sermon 
from  the  text,  "Ye  men  of  Galilee,  why  stand  ye  gazing  up 
into  heaven  ?  '^  ^ 

ELECTEICITY  AND  MAGNETISM 

By  the  side  of  Galileo,  "  the  originator  of  modern  physics," 
we  may  well  place  Gilbert,  "the  father  of  the  magnetic  phi- 
losophy."   William  Gilbert  (1540-1603)  of  Colchester,  county  of 

1  This  translation  is  taken  from  0.  Lodge,  Pioneers  of  Science,  1893, 
p.  106. 

2  A.  D.  White,  op.  ciL,  Vol.  I.,  p.  133. 


42  A   HISTORY    OF   PHYSICS 

Essex,  England,  studied  at  St.  John's  College,  Cambridge,  then 
travelled  on  the  Continent.  There,  as  well  as  in  England,  he 
"practised  as  a  physician  with  great  success  and  applause.'' 
He  was  appointed  by  Queen  Elizabeth  her  physician-in- 
ordinary,  and  she  settled  upon  him  an  annual  pension  for  the 
purpose  of  aiding  him  in  the  prosecution  of  his  philosophical 
studies.  His  first  investigations  were  in  chemistry ;  but  later, 
for  eighteen  years  or  more,  he  experimented  on  electricity  and 
magnetism.  In  1600  he  published  his  great  work,  the  De 
Magnete.  J.  E.  W.  Herschel  speaks  of  this  book  as  "full  of 
valuable  facts  and  experiments  ingeniously  reasoned  on."  It 
is  the  first  great  work  on  physical  science  produced  in  England. 
Galileo  pronounced  it  "  great  to  a  degree  that  is  enviable,"  but 
at  home  it  was  not  appreciated  so  highly.^  In  subsequent 
generations  the  book  was  quite  forgotten. 

Gilbert's  contempt  for  the  methods  of  the  schoolmen  crops 
out  everywhere  in  his  book.  In  fact,  his  criticisms  of  worthy 
predecessors  are  at  times  ungenerous.  He  withheld  his  work 
from  publication  for  many  years.  "  Why  should  I,"  says  he 
in  his  preface,  "  submit  this  noble  and  .  .  .  this  new  and  inad- 
missible philosophy  to  the  judgment  of  men  who  have  taken 
oath  to  follow  the  opinions  of  others,  to  the  most  senseless 
corrupters  of  the  arts,  to  lettered  clowns,  grammatists,  soph- 
ists, spouters,  and  the  wrong-headed  rabble,  to  be  denounced, 
torn  to  tatters,  and  heaped  with  contumely.  To  you  alone, 
true  philosophers,  ingenuous  minds,  who  not  only  in  books  but 
in  things  themselves  look  for  knowledge,  have  I  dedicated 
these  foundations  of  magnetic  science  —  a  new  style  of  phi- 
losophizing" (p.  xlix.).     Modern  philosophers  "must  be  made 

1  See  William  Gilbert  of  Colchester,  On  the  Loadstone  and 
Magnetic  Bodies,  and  on  the  Great  Magnet^  the  Earth,  trans,  by  P.  F. 
MoTTELAT,  London,  1893,  "Biographical  Memoir,"  pp.  ix-xxvii.  All  our 
references  will  be  to  this  edition  of  the  De  Magnete. 


ELECTRICITY   AND   MAGNETISM  43 

to  quit  the  sort  of  learning  that  comes  only  from  books,  and 
that  rests  only  on  vain  arguments  from  probability  and  upon 
conjectures"  (p.  47).  ''Men  of  acute  intelligence,  without 
actual  knowledge  of  facts,  and  in  the  absence  of  experiment, 
easily  slip  and  err"  (p.  82).  Gilbert  was  the  first  to  use  the 
terms  "electric  force,"  "electric  attraction,"  magnetic  "pole." 
Bodies  which  attract  in  the  same  way  as  amber  he  called 
"  electrics."  Metals  and  some  other  bodies  he  called  "  non- 
electrics,"  because  he  could  not  make  them  attract  by 
friction. 

Pupils  beginning  physics  sometimes  fail  to  discriminate 
between  magnetic  action  and  electric  attraction  or  repulsion. 
History  reveals  the  same  error  on  the  part  of  some  early 
writers.  The  differentiation  between  the  two  was  first  clearly 
made  by  the  Milanese  mathematician,  Hieronimo  Cardano 
(1501-1576).^  Gilbert  complains  of  those  who  "are  ignorant 
that  the  causes  of  the  loadstone's  movements  are  very  differ- 
ent from  those  which  give  to  amber  its  properties "  (p.  75). 
The  Italian  Baptista  Porta  had  taught  that  iron  rubbed  with 
diamond  turns  to  the  north,  as  if  it  had  been  rubbed  on  a 
loadstone.  To  this  Gilbert  says,  "We  made  the  experiment 
ourselves  with  seventy-five  diamonds  in  presence  of  many 
witnesses,  employing  a  number  of  iron  bars  and  pieces  of 
wire,  manipulating  them  with  the  greatest  care  while  they 
floated  in  water,  supported  by  corks ;  but  never  was  it 
granted  me  to  see  the  effect  mentioned  by  Porta"  (p.  218). 
Gilbert  wages  war  against  Cardan,  who  "asks  why  no  other 
metal  is  drawn  by  any  stone;  and  his  answer  is,  because  no 
other  metal  is  so  cold  as  iron ;  as  if,  forsooth,  cold  were  cause 
of  attraction,  or  iron  were  much  colder  than  lead,  which 
neither  follows  the  loadstone  nor  leans  toward  it.     But  this 

1  Consult  P.  Benjamin,  p.  249. 


44 


A   HISTOKY   OF   PHYSICS 


is  sorry  trifling,  no  better  than  old  wives'  gossip"  (p.  101). 
"  A  needle  turns  no  less  rapidly,  no  less  eagerly,  to  the  load- 
stone though  a  flame  intervenes  than  if  only  air  stands  be- 
tween" (p.  107).  He  then  makes  the  interesting  observation, 
"But  were  the  iron  itself  red-hot,  it  certainly  would  not  be 
attracted,"  though  it  will  be  "  as  soon  as  the  temperature  has 
fallen  somewhat"  (p.  107).  Some  modern  texts  give  the  ele- 
gant experiment  performed  by  Gilbert  of  magnetizing  an  iron 
bar  or  wire,  while  held  so  as  to  point  north  and  south,  by 
being  "stretched  or  hammered  or  pulled,"  or  by  being  ham- 
mered while  cooling  from  a  red  heat  (pp.  211,  212).  ' 

Gilbert's  experiments  on  terrestrial  magnetism  are  epoch 
making.     To  him  we  owe  the  "  new  and  till  now  unheard-of 

view  of  the  earth"  as  a 
great  magnet  (p.  64).^ 
Gilbert  followed  partly 
in  the  steps  of  Peregrinus 
and  used  a  little  load- 
stone formed  into  the 
shape  of  a  globe.  Plac- 
ing pivoted  needles  near 
this  magnetic  globe,  he 
observed  the  directive  and 
attractive  force  which  it 
exerted  upon  them.  In 
that  small  body  he  found 
mau}^  properties  of  the 
earth.  Hence  he  called  it  the  "terrella"  or  "little  earth." 
"  The  loadstone  possesses  the  actions  peculiar  to  the  globe,  of 


Fig.  5.    Gilbert's  Terkella. 


1  The  theory  that  the  earth  has  a  magnetic  pole  had  been  advanced  in 
1546  by  Gerhard  dleirator,  but  the  letter  on  this  subject  was  not  printed 
until  1869.    It  is  reprinted  in  Hellmann's  Neudrucke,  No.  10,  Berlin 
1898. 


ELECTRICITY   AND   MAGNETISM  45 

attraction,  polarity,  revolution,  of  taking  position  in  the  uni- 
verse according  to  the  law  of  the  whole"  (p.  6(5).  "Toward 
it,  as  we  see  in  the  case  of  the  earth,  magnetic  bodies  tend 
from  all  sides,  and  adhere  to  it "  (p.  67).  "  Like  the  earth  it 
has  an  equator,  .  .  .  [it]  has  the  power  of  direction  and  of 
standing  still  at  north  and  south  "  (p.  67). 

Since  the  earth  has  magnetic  poles,  it  follows  from  the  law 
of  magnetic  action  that  the  north-pointing  pole  of  a  needle  is 
the  south  pole;  "all  instrument  makers,  and  navigators,  are 
egregiously  mistaken  in  taking  for  the  north  pole  of  the  load- 
stone the  part  of  the  stone  that  inclines  to  the  north '^  (p.  27). 
Gilbert's  discovery  that  the  earth  is  a  huge  magnet  made  it 
easy  to  explain  why  the  needle  points  north.  Prior  to  Gilbert 
all  sorts  of  reasons  had  been  assigned.  "  The  common  herd  of 
philosophizers,  in  search  of  the  causes  of  magnetic  movements, 
called  in  causes  remote  and  far  away.  Martinus  Cortesius 
.  .  .  dreamt  of  an  attractive  magnetic  point  beyond  the 
heavens,  acting  on  iron.  Petrus  Peregrinus  holds  that 
direction  has  its  rise  at  the  celestial  poles.  Cardan  was 
of  the  opinion  that  the  rotation  of  iron  is  caused  by  the  star 
in  the  tail  of  Ursa  Major.  The  Frenchman  Bessard  thinks 
that  the  magnetic  needle  turns  to  the  pole  of  the  zodiac.  .  .  . 
So  has  ever  been  the  wont  of  mankind;  homely  things  are 
vile ;  things  from  abroad  and  things  afar  are  dear  to  them 
and  the  object  of  longing"  (p.  179). 

That  the  needle  does  not  point  true  north  and  south  was 
known  to  the  Chinese  as  early  as  the  eleventh  century.  That 
there  are  variations  in  declination  was  clearly  recognized  by 
Columbus  on  his  memorable  voyage  of  1492.  An  atlas  issued 
in  1436  by  Andrea  Blanco  was  formerly  believed  to  disclose 
the  knowledge  that  the  declination  is  not  everywhere  the 
same,  but  Bertelli  denies  him  this  knowledge  and  inter- 
prets  the  indicated   corrections   for  variation  in   a  different 


46  A   HISTORY   OF  PHYSICS 

manner.^  Columbus  was  certainly  tlie  first  to  make  known  a 
place  of  no  declination  which  he  found  not  far  from  the  island 
of  Corvo,  one  of  the  Azores.  Baptista  Porta  had  taught  that 
the  declination  varied  regularly  with  the  longitude,  so  that 
terrestrial  longitude  could  be  found  readily  from  the  observed 
declination.  Gilbert  had  data  on  hand  to  show  that  this  "  is 
false  as  false  can  be  "  (p.  251).  However,  Gilbert  himself  falls 
into  error  by  assuming  the  declination  at  any  one  place  to  be 
invariable,  and  by  presuming  that  the  magnetic  and  geographic 
equators  were  identical  and  that  lines  of  equal  dip  coincided 
with  the  geographic  parallels.  These  are  instances  showing 
that  the  propensity  to  speculation  without  checking  the  results 
by  "sure  experiment"  sometimes  secured  control  even  of 
Gilbert. 

The  existence  of  dip  is  usually  believed  to  have  been  dis- 
covered in  1576  by  Robert  Norman,  a  "  skilled  navigator  and 
ingenious  artificer  "  of  Bristol,  who  announced  the  new  fact  in 
a  treatise  of  1581,  entitled.  The  Newe  Attractiue.  To  Norman's 
treatise  was  added  a  supplement  prepared  in  1581  by  William 
Borough,  who  dwells  more  particularly  on  rules  for  finding 
the  declination.  Hellmann  attributes  the  discovery  of  dip  to 
Georg  Hartmann  in  1544,  but  admits  that  his  determination 
was  very  inaccurate.  Hartmann' s  letter  was  not  published 
till  1831.2 

Gilbert  was  a  strong  adherent  of  the  Copernican  system. 
One  object  of  his  book  was  to  furnish  additional  arguments 
in  support  of  the  new  doctrine.  His  experiments  exhibit 
throughout  painstaking  accuracy,  but  his  application  of  experi- 
mental   results    to    cosmology   was    inconclusive.      Thus,   he 

1  Bertelli,  Sulla  Epistola  di  P.  Peregrino,  Kome,  1868,  mem.  III.,  77  ; 
Benjamin,  p.  197. 

2  Hartmann's,  Norman's,  and  Borough's  interesting  papers  are  reprinted 
In  Hellmann's  Neudrucke^  No.  10. 


METEOROLOGY 


47 


endeavoured  to  prove  that  tlie  earth  rotated  because  of  its 
magnetic  quality.  No  doubt  these  unfortunate  speculations 
were  the  cause  of  the  undue  neglect  from  which  his  book 
suffered  so  long.  Yet  this  neglect  is  no  more  warranted  than 
would  be  that  of  Newton's  Opticks  because  of  his  advocacy 
of  the  emission  theory.  In  both  cases  the  ultimate  deductions 
are  wrong,  but  the  experimental  results  are  invaluable. 

METEOEOLOGY 

One  of  the  earliest  systematic  meteorological  records  is  that 
kept  in  the  years  1582-1597  by  the  astronomer  Tycho  Brake 
at  his  observatory  in 
Prague.^  Instruments 
for  weather  observations 
were  still  few.  The  wind- 
vane,  first  found  among 
the  Greeks,  was  placed 
in  Christian  Europe  on 
top  of  church-steeples, 
and  received  the  form 
of  a  cock,  because  that 
bird  was  the  emblem  of 
clerical  vigilance.^ 

About  1570  the  astron- 
omer Egnatio  Danti  had 
erected  in  Bologna  and  Florence  a  number  of  pendulum 
anemometers  (Fig.  6)  for  measuring  the  force  of  the  wind. 
In  modern  times  this  instrument  has  been  used  extensively 
in  Europe.  Its  first  invention  is  often  wrongly  attributed  to 
Robert  Hooke.^ 

1  G.  Hellmann,  Himmel  und  Erde,  Vol.  II.,  1890,  p.  113,  etc. 

2  Ibidem^  p.  119. 

^Ibidem,  p.  121 ;  Sprat,  Hist,  of  Boyal  Soc,  1667,  p.  173. 


Fig.  6. 


48  A   HISTORY   OF   PHYSICS 

The  earliest  known  hygroscope  is  described  in  the  works  of 
the  German  cardinal  Nicolaus  de  Cusa  (1401-1464).  He  says : 
"  If  yon  suspend  from  one  side  of  a  large  balance  a  large  quan- 
tity of  wool,  and  from  the  other  side  stones,  so  that  they  weigh 
equally  in  dry  air,  then  you  will  see  that  when  the  air  inclines 
toward  dampness,  the  weight  of  the  wool  increases,  and  when 
the  air  tends  to  dryness,  it  decreases."  The  Italians  attribute 
the  first  hygrometers  to  Da  Vinci.  About  the  middle  of  the 
sixteenth  century  Mizauld^  noticed  the  effect  of  moisture  on 
gut-strings.  This  has  since  been  used  repeatedly  in  the  design 
of  hygrometers.  About  the  same  time  Baptista  Porta  called 
attention  to  the  hygroscopic  properties  of  the  beards  of  wild 
oats.  He  saw  children  paste  to  a  beard  small  pieces  of  paper, 
which  would  bend  one  way  or  another,  according  as  the  air 
was  dry  or  moist.  In  the  early  part  of  the  seventeenth  century 
wild  oats  were  used  extensively  as  a  hygroscopic  substance. 

THE   INDUCTIVE   METHOD   OF   SCIENTIFIC 

INQUIRY 

The  necessity  of  observation  and  experiment  in  scientific 
research  was  emphasized  in  the  writings  of  Francis  Bacon 
He  was  a  man  of  extraordinary  literary  gifts,  and  his  works 
on  scientific  method  contain  many  bright  passages,  with  which 
popular  authors  delight  to  ornament  their  title-pages  and 
chapter-headings.  People  unacquainted  with  the  history  of 
scientific  progress  have  even  imagined  that  to  Francis  Bacon 
and  his  Novum  Organuni  are  principally  due  the  reawakening 
of  the  world,  the  overthrow  of  Aristotelian  physical  philoso- 
phy, and  the  introduction  into  science  of  the  inductive  method. 

1  Ephemerides  aeris  perpetuae,  Lutetiae,  1554,  p.  49 ;  Hellmann,  op. 
cit.,  p.  122. 


METHOD    OF   SCIENTIFIC    INQUIRY  49 

As  a  matter  of  fact,  Bacon  was  not  a  scientific  man ;  he  had 
little  practical  experience  in  experimentation ;  he  lacked  the 
scientific  instinct  to  pursue  in  detail  the  great  truth  that  nature 
must  be  studied  directly  by  observation  and  experiment.  He 
appears  to  have  rejected  the  Copernican  system,  and  he  treated 
with  contempt  the  researches  of  Galileo  and  Gilbert  —  the  two 
greatest  experimentalists  of  his  day.  Bacon  undertook  to 
give  an  infallible  rule  by  which  any  one  could,  with  persever- 
ance, make  scientific  discoveries.  We  "  must  analyze  Nature 
by  proper  rejections  and  exclusions,  and  then,  after  a  sufficient 
number  of  negatives,  come  to  a  conclusion  on  the  affirmative 
instances."  ^  He  thought  nature  could  be  studied  by  rule,  with- 
out the  aid  of  hypotheses  and  scientific  imagination.  His 
recipe  has  met  with  popular  applause,  but  has  never  been 
actually  followed  by  original  investigators  in  physical  or  chemi- 
cal laboratories.  Says  Professor  E.  Mach:  "1  do  not  know 
whether  Swift's  academy  of  schemers  in  Lagado,  in  which 
great  discoveries  and  inventions  were  made  by  a  sort  of  verbal 
game  of  dice,  was  intended  as  a  satire  on  Francis  Bacon's 
method  of  making  discoveries  by  means  of  huge  synoptic  tables 
constructed  by  scribes.  It  certainly  would  not  have  been  ill- 
placed."  ^ 

1  Novum  Organum,  I.,  Aphorism  CV. 

2  Monist,  Vol.  VI.,  1896,  p.  174.  Consult  further  0.  Lodge,  Pioneers 
of  Science,  pp.  136  et  seq.  ;  Jevons,  Principles  of  Science,  1892,  p.  507  ; 
P.  DuHEM,  L' Evolution  des  Theories  Physiques,  Louvain,  1896,  pp.  8-10; 
Justus  VON  Liebig,  Eeclen  und  Abhandlung,  Leipzig,  1874;  Draper, 
Hist,  of  the  Intell.  Develop,  of  Europe,  1875,  Vol.  IL,  p.  259;  Whewell, 
Vol.  I.,  p.  339. 


THE   SEVENTEENTH   CENTURY 

The  first  effects  of  the  Reformation  were  favourable  for  the 
progress  of  science  in  G-ermany.  But  during  and  after  the 
Thirty  Years'  War  (1618-1648)  civil  and  religious  strife,  as 
well  as  a  political  dismemberment  into  a  lax  confederation  of 
petty  despotisms,  ensued.  In  consequence,  science  almost  died 
out  in  Germany. 

In  France  the  ascension  of  Henry  TV.  to  the  throne,  and  the 
promulgation  of  the  Edict  of  Nantes  (1598),  somewhat  lessened 
religious  strife;  the  genius  of  the  French  people  began  to 
flourish.  At  the  time  when  the  blossoms  of  science  withered 
away  in  Germany,  they  were  budding  forth  in  France. 

In  Italy  the  fate  of  Galileo  dampened  scientific  enthusiasm, 
while  in  England,  where  religious  contention  never  fully 
engrossed  the  attention  of  the  people,  the  time  of  Gilbert  was 
followed  by  a  period  of  extraordinary  scientific  achievement. 

In  the  present  epoch  we  shall  contemplate  the  scientific 
labours  of  Torricelli  in  Italy,  Guericke  in  Germany,  Huygens 
in  Holland,  Pascal,  Mariotte,  and  Descartes  in  France,  Boyle, 
Hooke,  Halley,  and  Newton  in  England.  It  was  a  period  of 
great  experimental  as  well  as  theoretical  activity. 

MECHANICS 

As  we  have  seen,  Galileo,  in  his  explanation  of  the  path  of 
a  projectile  in  a  vacuum,  had  successfully  mastered  the  first 
and  the  second  law  of  motion.  Later  Descartes  wrote  on  me- 
chanics, but  he  hardly  advanced  beyond  Galileo.     Descartes's 

50 


MECHANICS  51 

statement  of  the  first  law  of  motion  (Piiricipia  PhilGsopMoe, 
1644)  was  an  improvement  in  form,  but  Ms  third  law  is  false 
in  substance.  The  motion  of  bodies  in  their  direct  impact 
was  imperfectly  understood  by  Galileo,  erroneously  given  by 
Descartes,  and  first  correctly  stated  by  Christopher  Wren,  John 
Wallis,  and  Christian  Huygens.  The  laws  of  motion  in  their 
present  form  were  first  given  by  Kewton  in  his  Principia. 

Descartes's  achievements  in  geometry  and  in  philosophy  are 
immeasurably  superior  to  those  in  physics.  He  was  a  meta- 
physician, and,  from  a  limited  amount  of  experimentation  or 
experience,  confidently  deduced  a  large  amount  of  inference, 
without  allowing  himself  to  be  disturbed  by  any  possible  dis- 
crepancy between  his  final  conclusions  and  the  actual  facts. 
He  had  no  appreciation  of  the  slow-going  process  of  Galileo.^ 

Says  Descartes :  "  Without  considering  the  first  causes  of 
nature,  he  (Galileo)  sought  only  for  the  causes  of  a  few  par- 
ticular effects  and  thus  built  without  a  foundation. ''  "  What 
Galileo  says  regarding  velocity  of  falling  bodies  in  a  vacuum 
has  no  foundation ;  he  should  have  told  what  gravity  is ;  had 
he  known  its  nature,  then  he  would  have  seen  that  there  is 
none  in  empty  space."  "  I  see  nothing  in  his  books  which  I 
envy  and  almost  nothing  which  I  would  acknowledge  as  my 
own.'' 2  According  to  his  own  a  priori  principles,  Descartes 
thought  he  could  easily  explain  all  that  Galileo  had  worked 
out,  while,  as  a  matter  of  fact,  Descartes  had  no  true  notion 
of  acceleration,  and  committed  errors  avoided  by  Galileo. 

There  arose  a  curious  dispute  between  the  Cartesians  and  the 

1  "  Of  the  mechanical  truths  which  were  easily  attainable  in  the  begin- 
ning of  the  seventeenth  century,  Galileo  took  hold  of  as  many,  and 
Descartes  of  as  few,  as  was  well  possible  for  a  man  of  genius." 
Whewell,  Vol.  I.,  p.  338. 

2  Descartes,  Lettres,  Vol.  II.,  Paris,  1659,  Letter  91,  p.  391 ;  Duheing, 
Krit.  Geschichte  d.  allgem.  Princ.  d.  Mechanik^  Leipzig,  1387,  pp.  106-108; 
Kastner,  Geschichte  d.  Mathematik^  Vol.  IV.,  pp.  22-26. 


52  A   HISTORY   OF   PHYSICS 

Leibnizians  on  the  measure  of  the  efficacy  of  a  moving  bod}/ 
Descartes  took  the  efficacy  to  be  proportional  to  the  velocity, 
Leibniz  took  it  to  vary  as  the  square  of  the  velocity}  The  con- 
troversy lasted  over  half  a  century,  until,  finally,  it  was  brought 
to  a  close  by  Jean-le-Rond  D'Alembert's  remarks  in  the  "pref- 
ace to  his  Dynamique,  1743,  though  before  this  date  Huygens's 
thought  on  this  subject  was  perfectly  clear.  The  long  dispute 
was  merely  one  of  words ;  both  views  were  correct.  The  effi- 
ciency of  a  body  in  motion  varies  as  its  velocity,  if  we  consider 
the  time.  A  body  thrown  vertically  upward  with  double  the 
velocity  ascends  twice  as  long  a  time.  The  efficiency  varies 
as  the  square  of  the  velocity,  if  we  consider  the  distance.  A 
body  thrown  vertically  upward  with  double  the  velocity  as- 
cends four  times  as  far.  The  reference  to  time  leads  to  what 
Descartes  called  the  "quantity  of  motion"  (our  "momen- 
tum"), mv,  and  makes  the  notion  oi  force  the  primary  concept. 
The  reference  to  the  distance  leads  to  the  expression  fs,  which 
makes  ivork  the  primary  notion.     The  former  view  made  ft= 

mv  the  fundamental  equation ;  the  latter  made  fs  =  -—  the 

fundamental  equation.     With  the  Cartesians  work  was  a  de- 
rived notion ;  with  the  Leibnizians  force  was  a  derived  notion.^ 

1  Acta  Eruditorum,  1686,  "  Demonstratio  erroris  memorabilis  cartesii," 
etc. 

2  For  the  term  ft  the  Frenchman  J.  B.  Belanger  proposed  in  1847  the 
name  impulse^  which  term  is  used  in  the  same  sense  by  J.  Clerk  Maxwell 
in  his  Matter  and  Motion.  Leibniz  (1695)  called  mv^  the  vis  viva  or 
living  force.  G.  G.  Coriolis  preferred  to  call  ^  mv^  the  vis  viva,  a  term 
now  called  kinetic  energy  by  the  English.  Coriolis  employed  the  name 
work  for  fs  and  was  sustained  in  this  usage  by  J.  V.  Poncelet,  who 
adopted  the  kilogramme-metre  as  the  unit  of  work.  Coriolis  and  Poncelet 
were  among  the  first  promoters  of  reform  in  the  teaching  of  rational 
mechanics.  See  Mach,  Science  of  Mechanics  (Ed.  McCormack),  pp.  271, 
272  ;  Marie,  Histoire  d.  Sciences  Math,  et  Phys.,  Vol.  XII.,  1888,  pp.  191, 
192. 


MECHANICS  53 

The  Cartesian  view,  followed  by  Newton  and  modern  writers 
of  elementary  text-books,  makes  force,  mass,  momentum,  the 
original  notions ;  the  Leibnizian  view,  followed  usually  by 
Huygens  and  by  the  school  of  Poncelet,  makes  work,  mass, 
vis  viva  (energy),  the  original  notions.-^  If  certain  modern 
thinkers  are  correct  in  afiSrming  the  objective  reality  of  kinetic 
energy  and  denying  the  objective  reality  of  force,  then  the 
Leibnizian  method  would  seem  to  be  the  more  philosophical.^ 

The  teacher  will  observe  that  those  parts  of  mechanics 
which  a  beginner  usually  finds  "  hard  to  learn "  are  the  parts 
which,  in  the  development  of  the  science,  were  difficult  to 
overcome.  Take,  for  instance,  the  difference  between  force 
and  energy,  or  the  concept  of  mass.  Early  writers,  such  as 
Galileo,  Descartes,  Leibniz,  Huygens,  had  no  clear  notion  of 
mass;  iceight  and  mass  were  taken  interchangeably;  these 
terms  were  one  and  the  same  thing.  The  real  distinction 
between  the  two  became  evident  when  it  was  discovered  that 
the  same  body  may  receive  different  accelorations  by  gravity 
on  different  parts  of  the  earth's  surface.  When  Jean  Richer 
in  1671  went  from  Paris  to  Cayenne  in  French  Guiana  to 
make  astronomical  observations,  he  found  that  his  pendulum 
clock,  which  in  Paris  kept  correct  time,  fell  daily  two  and  a 
half  minutes  behind  mean  solar  time.  The  pendulum  was 
shortened,  but  after  his  return  to  Paris  it  was  found  to  be  too 
short.^  The  keen-minded  Huygens  at  once  discerned  the  cause, 
and  found  a  partial  explanation  in  the  greater  centrifugal  ten- 
dency of  the  earth  in  Cayenne.*    The  distinction  between  mass 

1  Mach,  op.  cit.,  pp.  148,  250,  270-276  ;  H.  Klein.  Friiicipien  der 
3Iechanik,  Leipzig,  1872,  pp.  17,  18. 

2  Consult  P.  G.  Tait,  Becent  Advances  in  Physical  Science^  London, 
1885,  pp.  16,  343-368. 

3  Makie,  op.  cit.,  Vol.  v.,  1884,  p.  102. 

*  Huygens  calculated  that  centrifugal  action  renders  the  second's  pen- 
dulum at  the  poles  gig  shorter  than  at  the  equator,  and  that  the  cen- 


54  A    HISTORY    OF   PHYSICS 

and  weight  was  clearly  perceived  by  Newton  in  his  extension  ot 
the  laws  of  dynamics  to  heavenly  bodies.^  On  the  same  spot 
of  the  earth,  mass  and  weight  are  proportional  to  each  other. 
This  is  not  a  self-evident  fact ;  Newton  proved  it  in  course  of 
a  remarkable  series  of  tests  on  pendulums.  "  By  experiments 
made  with  the  greatest  accuracy,  I  have  always  found  the 
quantity  of  matter  in  bodies  to  be  proportional  to  their 
weight."  ^ 

The  mathematical  theory  of  the  pendulum  was  first  worked 
out  by  Huygens  in  his  De  horologio  oscillatorio  (Paris,  1673),  a 
work  that  ranks  second  only  to  the  Pnncipia  of  Newton. 
The  book  opens  with  a  description  of  pendulum  clocks.  Of 
his  new  theorems,  the  one  on  the  interchangeability  of  the 
point  of  suspension  and  centre  of  oscillation  has  found  its  way 
into  elementary  text-books. 

Before  proceeding  to  Newton's  discovery  of  the  law  of  gravi- 
tation, we  pass  in  brief  review  Descartes's  theory  of  vortices. 
After  the  overthrow  of  the  Ptolemaic  system  and  the  rejection 
of  the  ancient  crystalline  spheres,  the  puzzle  stared  philoso- 
phers in  the  face,  what  is  it  that  causes  the  planets  to  move  in 
their  orbits?  The  answer  given  in  Descartes's  theory  was 
eagerly  accepted.^  All  space  is  filled  with  a  fluid,  or  ether, 
the  parts  of  which  act  on  each  other  and  cause  circular  motion. 
Thus  the  fluid  was  formed  into  a  multitude  of  vortices  of  dif- 
ferent size,  velocity,  and  density.    There  is  an  immense  vortex 

trifugal  force  at  the  equator  is  ^^  of  the  absolute  weight  of  a  "body.  See 
HuTGE>-s,  Ursache  d.  Schwere,  trans,  by  E.  Mewes,  Berlin,  1896,  p.  34. 

1  Mach,  op.  cU.,  pp.  161,  251. 

2  Principia,  Booli  II.,  Prop.  XXIY.,  Cor.  7. 

3  It  is  an  interesting  fact  that,  by  his  theory,  Descartes  aimed  pri- 
marily to  reconcile  the  teachings  of  Copernicus  with  the  doctrine  of  the 
immobility  of  the  earth.  He  taught  "that  the  earth  is  at  rest  in  its 
heaven,  which  does  not  prevent  its  being  carried  along  with  it,  and  that 
it  is  the  same  with  all  the  planets." 


MECHANICS 


55 


around  the  sun,  carrying  in  its  whirl  the  earth  and  the  othei 
planets.  The  denser  bodies,  being  slower  and  less  subject  to 
centrifugal  action,  are 
forced  toward  the  sun, 
the  centre  of  the  vor- 
tex. Each  planet  is 
in  the  centre  of  an- 
other vortex  by  which 
the  ordinary  phenom- 
ena of  gravity  are  pro- 
duced. Still  smaller 
vortices  produce  cohe- 
sion between  parts  of 
a  body.  Figure  7  is 
Descartes's  diagram  of 
vortices  given  in  his 
Principia. 

This  theory  is  of 
interest,  because  it  is 
the  faith  on  which 
Kewton  was  brought 
up;  it  was  taught  in 
English  and  European 
universities.  In  1671 
Jacques  Rohault,  a  Car- 
tesian, wrote  his  Traite  de  Physique.  This  became  a  classic  text 
in  France,  and  was  taught  in  England  and  America.  Samuel 
darkens  translation  of  it  (1696)  was  used  as  a  text  at  Yale 
College  as  late  as  1743.-^  Clarke's  notes  to  the  original  text 
aimed  to  expose  the  fallacies  of  the  Cartesian  system,  and  to 
advance  Newtonian  views.     In  France  the  Newtonian  theory 


Fig.  7. 


1  Teaching  and  History  of  Math,  in  the  U.  S.,  Washington,  1890,  p.  30. 


56  A    HISTORY   OF   PHYSICS 

did  not  completely  dispel  the  belief  in  Descartes's  vortices 
until  the  middle  of  the  eighteenth  century.^ 

Descartes's  theory  of  vortices  can  hardly  be  ranked  among 
the  great  scientific  theories,  such  as  the  Ptolemaic  or  Coper- 
nican  system,  or  the  emission  theory  of  light.  Descartes  made 
no  attempt  to  reconcile  it  with  Kepler's  laws ;  in  fact,  it  did 
not  explain  a  single  phenomenon  satisfactorily.  JSTor  did  it 
lead  to  the  discovery  of  new  truths.  However,  it  referred 
planetary  motions  to  mechanical  causes.  Its  general  features 
were  easily  grasped,  for  a  whirlwind  or  an  eddy  of  water  at 
once  suggested  a  picture  to  the  mind.  Then,  too,  these  vor- 
tices helped  to  overthrow  the  Aristotelian  system.^ 

Isaac  Neivton  (1642-1727)  was  born  at  Woolsthorpe,  in 
Lincolnshire,  the  same  year  in  which  Galileo  died.  In  his 
twelfth  year  his  mother  sent  him  to  the  public  school  at 
Grantham,  where  he  began  to  show  decided  taste  for  me- 
chanical inventions.  He  constructed  a  water-clock,  a  windmill, 
a  carriage  moved  by  the  person  who  sat  in  it,  and  other  toys. 
He  entered  Trinity  College,  Cambridge,  in  1660.  Cambridge 
was  the  birthplace  of  Newton's  genius.  Among  the  physical 
works  read  by  him  while  an  undergraduate  are  Kepler's 
Optics  and  Barrow's  Lectures.  The  first  ideas  of  some  of  his 
greatest  discoveries  suggested  themselves  to  him  at  this  time. 

1  Voltahe,  who  visited  England  in  1727,  and  afterward  became  a 
stanch  supporter  of  Newton's  philosophy,  says,  "A  Frenchman  who 
arrives  in  London  finds  a  great  alteration  in  philosophy,  as  in  other 
things.  He  left  the  world  full  \_a  plenum'],  he  finds  it  empty.  At  Paris 
you  see  the  universe  composed  of  vortices  of  subtle  matter,  in  London  we 
see  nothing  of  the  kind,"  Whewell,  op.  cit.,  Vol.  I.,  p.  431.  We  can- 
Tiot  blame  the  Europeans  for  not  believing  in  "empty"  space.  In  this 
respect  Newton  himself  was  not  a  Newtonian.  See  Correspondence  of 
B.  Bentley,  Vol.  L,  p.  70;  Proc.  Boy.  Soc.  of  London,  Vol.  54.,  1893, 
p.  381. 

2  John  Playfaik,  "Dissertation  Fourth"  in  Encyclop.  Brit..,  Sth  ed., 
Vol.  L,  pp.  609,  610 ;  0.  Lodge,  Pioneers  of  Science^  pp.  152-156. 


MECHANICS  57 

In  1664  lie  made  some  observations  on  halos.^  In  1666,  "I 
began,"  he  says,  "to  think  of  gravity  extending  to.  the  orb  of 
the  moon,  .  .  .  and  thereby  compared  the  force  requisite  to 
keep  the  moon  in  her  orb  with  the  force  of  gravity  at  the 
surface  of  the  earth,  and  found  them  answer  pretty  nearly.''  ^ 

The  above  thoughts  on  gravitation  occurred  to  him  while 
he  was  at  his  home  in  Lincolnshire,  where  he  had  gone  to 
escape  the  plague  at  that  time  raging  in  Cambridge,  Pem- 
berton  gives  the  following  details :  "  As  he  sat  alone  in  the 
garden,  he  fell  into  a  speculation  on  the  power  of  gravity ;  that 
as  this  power  is  not  found  sensibly  diminished  at  the  remotest 
distance  from  the  centre  of  the  earth  to  which  we  can  rise, 
neither  at  the  tops  of  the  loftiest  buildings,  nor  even  on  the 
summits  of  the  highest  mountains  ;  it  appeared  to  him  reason- 
able to  conclude,  that  this  power  must  extend  much  farther 
than  was  usually  thought ;  why  not  as  high  as  the  moon,  said 
he  to  himself  ?  And  if  so,  her  motion  must  be  influenced  by 
it;  perhaps  she  is  retained  in  her  orbit  thereby."^  It  was 
conjectured  by  Newton,  as  also  by  Hooke,  Huygens,  Halley, 
Wren,  and  others,  that  if  Kepler's  third  law  (the  square  of  the 
time  of  revolution  of  each  planet  is  proportional  to  the  cube  of 
its  mean  distance  from  the  sun)  was  true,  then  the  attraction 
between  the  earth  and  other  members  of  the  solar  system 
varied  inversely  as  the  square  of  the  distance.  The  accuracy 
of  Kepler's  third  law  was  doubted  at  that  time.  To  show  that 
the  above  conjecture  was  true  required  the  genius  of  Newton. 

1  Newton,  Opticks,  London,  1704,  Book  II.,  Part  IV.,  obs.  13,  p.  111. 

^Portsmouth  Collection^  Sect.  I.,  Division  XI.,  No.  41;  W.  W.  R. 
Ball,  An  Essay  on  JSfeioton'' s  ^'-  Principia,''''  London,  1893,  p.  7. 

3  Pembertox,  View  of  Sir  Isaac  Newton'' s  Philosophy,  London,  1728 ; 
W,  W.  R.  Ball,  op.  cit.,  p.  9.  The  well-known  anecdote  that  the  idea  of 
universal  gravitation  was  suggested  to  Newton  by  tiie  fall  of  an  apple  is 
usually  considered  legendary,  but  Ball  argues  in  its  favour,  and  gives  the 
authorities  bearing  on  it,  pp.  11,  12. 


58  A  HISTORY   OF   PHYSICS 

According  to  the  old  account  of  the  discovery,  Newton  in 
1666  based  his  estimate  of  the  earth's  radius  on  the  suppo- 
sition that  there  were  60  miles  to  a  degree  of  latitude.  This 
verified  the  law  of  inverse  squares  only  approximately  and 
threw  doubt  upon  Newton's  speculations.  About  1684  he 
obtained  from  Jean  Picard's  measurement  of  an  arc  of  the 
meridian  (69^  miles  to  a  degree)  a  more  accurate  value  for 
the  earth's  radius.  Taking  this  corrected  value,  the  law  of 
inverse  squares  was  verified. 

More  recent  research  renders  it  highly  improbable  that 
Newton  remained  long  unacquainted  with  the  fact  that  the 
estimate  of  60  miles  was  too  small.  Norwood's  Seaman's 
Practice,  1636,  gave  the  more  correct  value  of  69-|-  miles  to  the 
degree.  Snell  had  given  nearly  the  same  result  in  1617,  and 
this  was  referred  to  in  Varenius's  Geography,  an  edition  of 
which  was  prepared  in  1672  by  Newton  himself.^  Neverthe- 
less, Newton  deferred  undertaking  a  recalculation  for  many 
years.  Why  this  delay?  The  astronomer,  J.  C.  Adams, 
examined  a  great  mass  of  unpublished  letters  and  manuscripts 
of  Newton  forming  the  Portsmouth  Collection  (which  remained 
private  property  until  1872,  when  its  owner  placed  it  in  the 
hands  of  the  University  of  Cambridge),  and  arrived  at  the 
opinion  that  Newton's  difB.cultieg  were  of  a  different  nature ; 
that  the  numerical  verification  was  fairly  complete  in  1666, 
but  that  Newton  had  not  been  able  to  determine  what  the 
attraction  of  a  spherical  body  upon  an  external  point  would 
be.  His  letters  to  Halley  show  that  he  did  not  suppose  the 
earth  to  attract  as  though  all  its  mass  were  concentrated  into 
a  point  at  the  centre.  He  could  not  assert,  therefore,  that  the 
assumed  law  of  gravity  was  verified  by  the  figures,  though  for 
long  distances  he  might  have  claimed  that  it  yielded  close 

1  R.  T.  Glazebrook,  article  "Newton  "  in  Die.  Nat.  Biog, 


MECHANICS 


5S 


approximations.  When  Halley  visited  Newton  in  1684,  he 
requested  Newton  to  determine  what  the  orbit  of  a  planet 
would  be  if  the  law  of  attraction  were  that  of  inverse  squares. 
Kewton  had  solved  a  similar  problem  for  Hooke  in  1679,  and 
replied  at  once  that  it  was  an  ellipse.  After  Halley's  visit, 
Newton,  with  Picard's  new  value  for  the  earth's  radius, 
reviewed  his  early  calculation,  and  was  able  to  show  that,  if 
the  distances  between  the  bodies  in  the  solar  system  were  so 
great  that  the  bodies  might  be  considered  as  points,  then  their 
motions  were  in  accordance  with  the  assumed  law  of  gravita- 
tion. In  1685  he  completed  his 
discovery  by  showing  that  the 
sphere  whose  density  at  any 
point  depends  only  on  the  dis- 
tance from  the  centre,  attracts  an 
external  particle  as  though  its 
whole  mass  were  concentrated  at 
the  centre.^  It  was  thus  proved 
that  the  force  of  attraction  be- 
tween two  spheres  is  the  same  as 
it  would  be  if  the  mass  of  each 
sphere  were  concentrated  at  its  cen- 
tre.    "No  sooner,"  says  Glaisher, 

*'had  Newton  proved  this  superb  theorem — and  we  know 
from  his  own  words  that  he  had  no  expectation  of  so  beau- 
tiful a  result  till  it  emerged  from  his  mathematical   investi- 


1  Consult  theorems  in  Principia,  Book  I.,  Sec.  XII.,  also  Book  III., 
Prop.  VIII.  For  further  details  on  the  discovery  of  the  law  of  gravita- 
tion, consult  W.  W.  R.  Ball,  op.  cit.  ;  J.  W.  L.  Glaisher,  "Bicente- 
nary Address,"  Cambridge  Chronicle^  April  20,  1888.  We  have  used  also 
Ball,  Hist,  of  Math.,  1888,  pp.  295-297.  Rosenberger's  Isaac  Newton 
und  seine  Physikalischen  Principien  is  worthy  of  reference,  even  though 
the  author  made  no  use  whatever  of  the  information  obtained  through 
the  Portsmouth  Collection. 


60  A   HISTOKY   OF   PHYSICS 

gation  —  than  all  the  meclianism  of  the  universe  at  once  la^ 
spread  before  him." 

We  proceed,  with  aid  of  Fig.  S,  to  explain  Newton's  calcula- 
tion. Geodetic  measurements  gave  the  circumference  of  the 
earth  as  123,249,600  Paris  feet  {Principia,  Book  III.,  Prop. 
IV.).  The  moon's  mean  distance  is  about  60  times  the  earth's 
radius.  Hence,  the  moon's  orbit,  assumed  to  be  circular,  is 
123,249,600  x  60  =  7,394,976,000  feet.  The  moon  revolves 
around  the  earth  once  in  27  d.  7  h.  43  m.,  or  in  39,343  min- 
utes. Hence  her  orbital  velocity  is  7,394,976,000  ^  39,343  = 
187,961.67  ft.  per  minute.  Let  the  arc  JOf'  represent  this 
velocity,  where  M  is  the  moon's  position  in  its  orbit  and  E  is 
the  earth's  centre.  Then,  evidently,  NM\  which  for  a  very 
small  angle,  MEM',  nearly  equals  MO,  represents  the  distance 
the  moon  falls  per  minute  toward  the  earth.     Since 


MM'  =  MP'  MO,  (Book  I.,  Prop.  lY.,  Cor.  9), 
we  get 

MO  =MM'^  -^  MP=  ISyL  ft.  per  minute,  nearly. 

Since  ME  equals  60  radii  of  the  earth,  the  distance  a  body 
falls  per  minute  on  the .  earth's  surface  should  be,  by  the 
law  of  inverse  squares,  60^  x  15^^  ft.  per  minute,  or  15y12-  ft. 
per  second.  More  accurately  it  is  "15  ft.,  1  inch,  and  1 
line  |-."  Now,  pendulum  experiments  made  by  Huygens  gave 
as  the  distance  per  second  through  which  a  body  falls  from 
rest  at  Paris  as  "15  ft.,  1  inch,  1  line  -J"  (Book  III.,  Prop.  IV.). 
Hence  the  law  of  inverse  squares  is  proved  to  be  true. 

In  the  scholium  to  Prop.  IV.,  Book  I.,  Newton  acknowledges 
his  indebtedness  to  Huygens  for  the  laws  of  centrifugal  force 
employed  in  the  above  calculation. 

When  Newton  presented  his  Principia  to  the  Eoyal  Society, 
Robert  Hooke  (1635-1703)  claimed  the  law  of  inverse  squares 
for  himself,    Newton's  reply  is  contained  in  a  letter  to  Edmund 


MECHAKICS  61 

Halley.^  The  Principia  was  publislied  in  1687  under  the 
direction  and  at  the  expense  of  Halley. 

Though  the  law  expressing  the  variation  in  the  intensity  of 
gravitational  attraction  became  known  over  three  centuries 
ago,  and  though  scientific  discovery  since  that  time  has  been 
more  rapid  than  ever  before,  Ave  are  still  unable  to  explain 
what  causes  a  stone  to  fall  to  the  ground.  This  is  indeed  a 
strange  fact  in  the  progress  of  science.  That  the  earth  and  the 
moon  act  upon  each  other  through  empty  space,  without  the 
aid  of  some  medium  between  them  or  surrounding  them, 
modern  physicists  find  it  difficult  to  believe.  No  little  interest 
attaches  to  the  question  as  to  ISTewton's  view  on  this  subject. 
Did  he  believe  in  "  action  at  a  distance,"  or  in  the  idea  that 
matter  can  act  where  it  is  not?  In  a  letter  to  Bentley  he 
says : 

"  That  gravity  should  be  innate,  inherent,  and  essential  to 
matter,  so  that  one  body  may  act  upon  another  at  a  distance 
through  a  vacuum  without  the  mediation  of  anything  else,  by 
and  through  which  their  action  and  force  may  be  conveyed 
from  one  to  another,  is  to  me  so  great  an  absurdity  that  I 
believe  no  man  who  has  in  philosophical  matters  a  competent 
faculty  of  thinking  can  ever  fall  into."  ^ 

Yet  the  opposite  belief  has  sometimes  been  ascribed  to 
Newton.  The  doctrine  of  action  at  a  distance  has  for  its 
author,  not  Newton,  but  Roger  Cotes  (1682-1716),  who  edited 
the  second  edition  of  the  Principia  in  1713  and  asserted  the 
doctrine  in  his  preface.     When  later  the  Newtonian  philosophy 

1  See  letter  in  Ball,  op.  cit.,  p.  155. 

2  Corresp.  ofB.  Bentley,  Vol.  I.,  p.  70  ;  Proc.  ofBoyal  Soc.  of  London, 
Vol.  54, 1893,  p.  381.  For  other  passages  from  Newton  favouring  an  ether- 
hypothesis,  see  his  Opticks,  Queries  18,  22  ;  also  Phil.  Trans.  Abr.,  Vol.  I., 
p.  145,  Nov.,  1672;  Birch,  Hist,  of  Boyal  Society,  Vol.  III.,  p.  249, 
1675. 


62  A   HISTORY    OF   PHYSICS 

gained  ground  in  Europe,  it  was  tlie  opinion  of  Cotes,  rather 
than  that  of  Newton,  which  prevailed.^ 

Proceeding  to  the  mechanics  of  liquids  and  gases,  we  begin 
with  researches  on  liquid  pressure  by  Blaise  Pascal  (1623- 
1662),  who  is  celebrated  not  only  as  a  precocious  mathemati- 
cian and  as  the  author  of  the  Provincial  Letters,  but  also  as  a 
physicist.  He  was  born  at  Clermont  in  Auvergne.  In  his 
brief  Traite  de  Vequilibre  des  liqueurs,^  written  in  1653  and  first 
published  in  1663,  one  year  after  his  death,  he  enunciates  the 
law,  known  as  "Pascal's  Law,"  that  the  pressure  exerted  upon 
a  liquid  is  transmitted  undiminished  in  all  directions  and  acts 
with  the  same  force  on  all  equal  surfaces  in  a  direction  at  right 
angles  to  them.  He  shows  by  experiments  identical  with 
those  carried  out  in  our  modern  laboratories  with  Masson's 

1  C.  Maxwell,  Lecture  "  On  Action  at  a  Distance,"  Nature,  Vol.  VII., 
1872-73,  p.  825.  Cotes's  preface  is  given  in  Sir  Isaac  Newtoii's  Principia 
reprinted  for  Sir  William  Thomson  and  Hugh  Blackburn,  Glasgow,  1871. 
Our  inability  to  explain  gravity  is  not  due  to  want  of  attempts.  The 
first  important  effort  in  this  line  was  made  by  C.  Huygens  in  his  Discours 
sur  la  cause  de  la  pesanteur,  part  of  which  was  written  after  the  appear- 
ance of  Newton's  Principia  in  1687.  A  German  translation  of  the  Dis- 
cours has  been  brought  out  by  Rudolf  Mewes,  Berlin,  1896.  A  mechani- 
cal gravitation  theory  was  advanced  by  C.  Le  Sage,  born  at  Geneva  in 
1724.  See  Le  Sage,  "  Lucr^ce  Newtonien,"  3Iemoires  de  VAcademie  des 
Sciences,  Berlin,  1782,  pp.  404-432.  He  teaches  that  gravity  is  caused  by 
streams  of  atoms  coming  in  all  directions  from  space.  Later  speculations 
on  the  cause  of  gravity  were  made  by  Clerk  Maxwell,  Lord  Kelvin,  C.Isen- 
Tcrahe,  Bernhard  Biemann,  Leonhard  Eider,  N.  v.  Belling shausen,  Tolver 
Preston,  Adalbert  RysdnecJc,  Paul  du  Bois-Beymond,  Vaschy,  Schramm, 
Anderson,  M'oller,  and  others.  For  critical  and  historical  summaries,  see 
C.  IsENKRAHE,  "  Uebcr  die  Zurtickfiihrung  der  Schwere  auf  Absorption  " 
vo-Zeitsch.  f.  3Iath.  and  Physik,  1892,  SuppL,  pp.  163-204;  S.  Tolver 
Preston,  "  Comparative  Review  of  some  Dynamical  Theories  of  Gravita- 
tion," Philosophical  Magazine,  (5)  Vol.  39,  1895,  pp.  145  et  seq.;  W.  B. 
Taylor,  "Kinetic  Theories  of  Gravitation,"  Smithsonian  Beport,  1876, 
pp.  205-282. 

2  (Euvres  Completes  de  Blaise  Pascal,  Paris,  1866,  Vol.  III.,  pp.  88-9& 


MECHANICS  6S 

apparatus  that  pressure  against  a  surface,  in  virtue  of  the 
weight  of  the  liquid,  depends  simply  upon  its  depth.  Several 
vessels  of  different  shapes  having  movable  bottoms  of  equal 
areas  are  suspended,  one  after  the  other,  from  one  arm  of  a 
balance.  The  vessels  are  filled  with  water  to  such  a  height 
that  the  pressure  is  just  sufficient  to  force  down  the  bottom 
and  raise  a  weight  on  the  other  arm  of  the  balance.  Pascal 
also  takes  two  sliding  plugs  or  pistons  pressing  against  a  fluid 
in  a  closed  vessel,  the  surface  of  the  first  being  one  hundred 
times  greater  than  the  surface  of  the  other ;  the  force  of  one 
man  acting  at  the  first  plug  will  balance  the  force  of  one  hun- 
dred men  at  the  other.  "  Hence  it  follows  that  a  vessel  full 
of  water  is  a  new  principle  of  mechanics  and  a  new  machine 
for  multiplying  forces  to  any  degree  we  choose.'^  ^ 

Except  the  telescope,  no  scientific  discovery  of  the  seven- 
teenth century  excited  wonder  and  curiosity  to  a  greater  degree 
than  did  the  experiments  with  the  barometer  and  air-pump. 
Chance  expressions  that  air  has  weight  are  already  found  in 
Aristotle  and  Plato,  but  nothing  was  hnoimi  till  the  time  of 
Galileo  and  Torricelli.  A  great  deal  of  vague  speculation  was 
indulged  in  regarding  the  vacuum.  Aristotle  believed  that  a 
vacuum  could  not  exist,  and  as  late  a  writer  as  Descartes  held 
the  same  view.  Por  two  thousand  years  philosophers  spoke 
of  the  horror  that  nature  has  for  empty  space,  —  the  horror 
vacui,  —  as  if  inanimate  objects  could  have  feeling.  Because 
of  this  horror,  nature  was  said  to  prevent  the  formation  of 
a  vacuum  by  laying  hold  of  anything  near  by  and  with  it 
instantly  filling  up  any  vacated  space.  Even  Galileo  could 
not  quite  free  himself  from  this  unphilosophical  doctrine.  He 
was  astonished  when  told  that  a  suction-pump  with  a  very 
long  suction-pipe,  which  had  just  been  constructed,  would  not 

1  CEuvres  Completes  de  Blaise  Pascal^  Paris,  1866,  Vol.  III.,  p.  86. 


64  A  HISTORY   OF   PHYSICS 

raise  water  higlier  than  about  thirty-three  feet.  He  remarked 
that  the  Jwrror  vacui  was  a  force  which  had  its  limitations  and 
could  be  measured.  That  air  has  weight  he  convinced  himself 
by  the  difference  in  weight  of  a  glass  balloon  filled  with  air 
under  ordinary  pressure  and  then  with  air  under  high  press- 
ure.^ He  estimated  the  density  of  air  to  be  400  times  less 
than  that  of  water. 

Thus  Galileo  knew  (1)  that  air  has  weight;  he  knew  also 
(2)  what  the  "  resistance  to  a  vacuum  "  was  when  measured  by 
the  height  of  a  water  column,  and  also  when  determined  by 
the  weight  against  a  piston.  But  the  two  ideas  dwelt  sepa- 
rately in  his  mind.^  It  remained  for  his  pupil  Torricelli  to 
vary  the  experiments,  to  unite  and  interweave  the  two  ideas, 
and  to  place  air  in  the  list  of  pressure-exerting  fluids. 

EvangeUsta  Torricelli  (1608-1647)  began  his  mathematical 
studies  in  a  Jesuit  school,  and  continued  them  under  Benedict 
Castelli  at  Eome.  He  made  himself  familiar  with  Galileo's 
writings,  and  published  some  articles  on  mechanics.  Galileo 
was  anxious  to  become  acquainted  with  the  author  of  these 
tracts  and  pressed  Torricelli  to  join  him  at  Florence.  He 
accepted  the  invitation,  and  it  is  said  that  his  society  and 
conversation  contributed  greatly  to  soothe  the  last  days  of  the 
blind  physicist.  Galileo  died  three  months  later.  Galileo's 
patron,  the  Grand  Duke  of  Tuscany,  made  Torricelli  Galileo's 
successor  as  professor  of  mathematics  at  the  Accademia. 

Torricelli  devised  the  scheme  of  determining  the  resistance 
of  a  vacuum  by  a  vertical  column  of  mercury,  which  he  ex- 
pected to  be  about  y^  the  length  of  the  corresponding  water 
column.  The  "  Torricellian  experiment ''  was  carried  out  in 
1643   in  Florence  by   Vi^icenzo    Viviani  (1622-1703),  who  at 

1  Ostwald^s  Klassiker,  No.  11,  p.  71.  See  also  Mach,  op.  cit.,  pp.  112- 
114. 

2  Mach,  in  Monist,  Vol.  6,  1896,  p.  170. 


MECHANICS  65 

seventeen  had  become  a  pupil  of  Galileo,  and  was  now  study- 
ing under  the  direction  of  Torricelli. 

Torricelli  never  published  an  account  of  his  research.  He 
was  at  this  time  too  deeply  absorbed  in  mathematical  investi- 
gations on  the  cycloid,  and  he  died  a  few  years  later.  How- 
ever, he  described  his  experiments  in  two  letters  of  1644,  to 
his  friend,  M.  A.  Eicci,  in  Rome,  and  these  are  extant.^  He 
says  that  the  aim  of  his  investigation  was  '^  not  simply  to  pro- 
duce a  vacuum,  but  to  make  an  instrument  which  shows  the 
mutations  of  the  air,  now  heavier  and  dense,  and  now  lighter 
and  thin."  ^ 

In  1644  Ricci  wrote  a  letter,  describing  the  Torricellian  ex- 
periment to  P^re  Mersenne,  in  Paris,  who,  by  his  extensive 
correspondence,  acted  as  an  intermediary  between  scientific 
men.  The  news  created  a  sensation  among  French  savants. 
But  the  experiment  was  not  repeated  in  France  until  the  sum- 
mer of  1646  (by  Pierre  Petit,  of  E,ouen,  in  conjunction  with 
Pascal) ;  as  no  suitable  glass  tubes  were  available  before  that 
date. 

The  account  of  the  Italian  experiment  which  reached  Pascal 
must  have  been  quite  incomplete,  for  he  found  it  necessary  to 
reflect  on  the  phenomenon  independently.  He  concludes  "  that 
the  vacuum  is  not  impossible  in  nature,  and  that  she  does  not 
shun  it  with  so  great  a  horror  as  many  imagine."  ^ 

1  They  were  first  published  in  1663.  See  a  recent  reprint  by  G. 
Hellmann,  NeudrucTce,  No.  7. 

2  At  the  close  of  the  letter,  he  says,  "  My  principal  object  is,  therefore, 
not  altogether  successful  .  .  .  because  the  level  [of  the  mercury]  .  .  . 
changes  for  another  cause  which  I  never  thought  of,  namely,  by  the  heat 
and  cold,    and  that  very  appreciably."     Yet   only   since  the  time   of 

,Amontons  (1704)  has  it  been  thought  necessary  to  make  corrections  for 
temperature.     See  G.  Hellmann,  op.  cit.,  pp.  16,  (3). 

3  "Nouvelles  expMences  touchant  le  vide,*'  (Euvres  Compl.  de  B. 
Pascal,  Paris,  1866,  Vol.  III.,  p.  1.  See  also  his  Traite  de  la  pesanteur 
de  la  masse  de  Vair,  pp.  98-129. 


66  A   HISTORY   OF   PHYSICS 

Pascal  reasoned  that,  if  the  mercury  column  was  held  up 
simply  by  the  pressure  of  the  air,  then  the  column  ought  to  be 
shorter  at  a  high  altitude.  He  tried  it  on  a  church-steeple  in 
Paris,  but  desiring  more  decisive  results,  he  wrote  to  his 
brother-in-law  to  try  the  experiment  on  the  Puy  de  Dome,  a 
high  mountain  in  Auvergne.  There  was  a  difference  of  three 
inches  in  the  height  of  the  mercury,  "  which  ravished  us  with 
admiration  and  astonishment."  Pascal  repeated  the  Torri- 
cellian experiment,  using  red  wine  and  a  glass  tube  forty-six 
feet  long.  (Evidently  glass  tubing  had  become  more  plentiful.) 
He  experimented  with  the  siphon,  and  explained  its  theory. 
A  balloon,  half  full  of  air,  was  found  to  appear  inflated  on 
being  taken  up  on  a  mountain,  and  to  flatten  again,  gradually, 
during  the  descent. 

The  doctrine  of  the  horror  vacui  was  overthrown  through 
experimental  research  in  Italy  and  France.  A  repetition  of 
the  process  took  place  in  Germany.  The  early  work  of  the 
German  investigator  was  carried  on  independently.  Otto  von 
Guericke  (1602-1686)  came  of  a  prominent  family  in  Magde- 
burg. He  studied  at  German  universities,  also  at  Leyden,  and 
then  travelled  in  England  and  Prance.  In  the  course  of  the 
Thirty  Years'  War  Magdeburg  was  devastated  in  1631,  and 
Guericke  and  his  family  barely  escaped  with  their  lives.  Later 
he  earned  a  livelihood  as  engineer  in  the  army  of  Gustavus 
Adolphus.     In  1646  he  became  burgomaster  of  Magdeburg. 

The  disputations  regarding  the  vacuum  made  him  curious  to 
find  out  the  facts  experimentally.  Says  he,  "Oratory,  ele- 
gance of  words,  or  skill  in  disputation  avails  nothing  in  the 
field  of  natural  science."  In  1663  he  completed  the  manu- 
script for  his  work  De  vacuo  spatio,  but  it  was  not  published 
until  1672.1 

1  The  work  consists  of  seven  books,  of  which  the  third  contains  his 
own  experimentgi  and  has  been  recently  brought  out  in  German  transla- 


MECHANICS 


67 


Guericke  first  took  a  tiglit  wine-cask,  full  of  water,  and 
attempted  to  remove  the  liquid  with  a  brass  pump  applied 
to  the  cask  below.  But  the  bands  and  iron  screws  holding 
the  pump  to  the  cask  gave  way.  It  was  attached  more 
securely,  then  three  strong  men  pulling  at  the  piston  at 
last  succeeded  in  drawing  out  the  water.     Thereupon  a  noise 


Fig.  9. 


was  heard,  as  if  the  residual  water  within  were  boiling  vio- 
lently, and  this  continued  until  air  had  replaced  the  water 
pumped  out. 


tion  in  OstwalcVs  Klassiker,  No.  59.  The  first  accounts  of  Guericke's 
air-pump  and  experiments  were  published  by  Kaspar  Schott  in  his 
Mechanica  hydraiilico-pneumatica,  1657,  and  in  his  Technica  cimosa, 
1664.  Througli  the  publication  of  1657  Robert  Boyle  became  acquainted 
with  Guericke's  experiments. 


68  A   HISTORY   OF   PHYSICS 

The  leaky  wooden  cask  was  replaced  by  a  copper  globe, 
then  water  and  air  were  drawn  out  as  before.  At  first  the 
piston  moved  easily,  but  later  the  strength  of  two  men  could 
hardly  move  it,  when  "  suddenly  with  a  loud  clap  and  to  the 
terror  of  all "  the  sphere  collapsed.  A  more  massive  and 
more  exactly  spherical  metallic  vessel,  Eig.  9,  was  secured 
and  exhausted.  "On  opening  the  stop-cock  the  air  rushed 
with  such  force  into  the  copper  globe,  as  if  it  wanted  to  drag 
to  itself  a  person  near  by.  Though  you  held  your  face  at  con- 
siderable distance,  your  breath  was  taken  away ;  indeed,  you 
could  not  hold  your  hand  over  the  stop-cock  without  danger 
that  it  would  be  violently  forced  down." 

Guericke  next  invented  air-pumps,  the  first  form  of  which 
is  illustrated  in  Fig.  10.  Its  tap  with  the  stop-cock  was 
detachable,  so  that  objects  to  be  experimented  upon  might 
be  placed  in  the  receiver.  As  an  extra  precaution  against 
leakage,  the  stop-cock  was  made  to  stand  under  water  which 
was  poured  into  the  conical  vessel.  Numerous  experiments 
were  made  with  this  pump.  A  clock  in  a  vacuum  cannot  be 
heard  to  strike ;  a  flame  dies  out  in  it ;  a  bird  opens  its  bill 
wide,  struggles  for  air,  and  dies ;  fishes  perish ;  grapes  can  be 
preserved  six  months  in  vacuo.  A  long  tube,  connected  with 
an  exhausted  globe  above  and  dipping  in  water  below,  was  his 
water  barometer.  He  explains  the  rise  of  the  water  in  the 
tube  by  the  pressure  of  the  air.  He  observes  fluctuations  in 
the  height  of  the  water  column  and  uses  the  instrument  for 
weather  predictions.  A  miniature  man  of  wood,  floating  on 
the  water,  moved  up  and  down  inside  the  tube  and  by  his 
finger  indicated  the  pressure  of  the  air  at  any  moment. 
Guericke's  experiment  cf  weighing  a  receiver,  first  when  full 
of  air  and  again  when  exhausted,  has  held  its  place  in  ele- 
mentary books.  The  same  is  true  of  his  "  Magdeburg  hemi- 
spheres.'^    He  constructed  such  hemispheres,  about  1.2  ft.  in 


MECHANICS 


69 


diameter,  and  made  a  test  in  1654  at  Eegensburg  before  the 
Reichstag  and   Emperor   Ferdinand   III.      According   to  his 


ICONISMUS  6 


Cap.  4   Lib.  3 


Fig.  10. 


calculations,  a  force  of  2686  pounds  was  needed  to  overcome 
the  atmospheric  pressure  holding  the  hemispheres  together. 


70  A   HISTORY   OF   PHYSICS 

They  were  pulled  apart  only  after  applying  sixteen  horses, 
four  pairs  on  each  hemisphere.  His  book  contains  a  large 
engraving,  naively  illustrating  how  the  experiment  was 
made. 

On  that  occasion  Guericke  made  other  experiments,  and  he 
happened  to  assert  that  if  you  were  to  blow  your  breath  into 
a  large  exhausted  receiver,  you  would  that  moment  breathe 
your  last.  The  truth  of  this  being  doubted,  he  illustrated 
the  power  of  "  suction "  by  a  new  experiment.  A  cylinder 
of  a  large  pump  had  a  rope  attached  to  its  piston,  which  led 
over  a  pulley  and  was  divided  into  branches  on  which  twenty 
or  thirty  men  could  pull.  As  soon  as  the  cylinder  was  con- 
nected with  an  exhausted  receiver,  the  piston  was  suddenly 
pushed  down  by  the  atmospheric  pressure  and  the  men  at 
the  ropes  were  thrown  forward. 

It  was  on  this  occasion  that  Guericke  heard  for  the  first  time 
of  the  Torricellian  experiments  made  eleven  years  earlier.^ 

In  England  the  mechanics  of  the  air  was  first  studied  by 
Robert  Boyle  (1627-1391).  He  was  born  at  Lismore  Castle 
in  Ireland.  In  his  autobiography  he  speaks  of  "  his  acquaint- 
ance with  some  children  of  his  own  age,  whose  stuttering  habi- 
tude he  so  long  counterfeited  that  at  last  he  contracted  it ; '' 
diverse  cures  "were  tried  with  as  much  successlessness  as 
diligence."  ^  After  spending  nearly  four  years  at  Eton  College, 
he  left  in  1638  for  the  Continent.  At  Geneva,  one  night,  a 
terrific  thunder-storm  made  him  fear  that  the  day  of  j-udg- 

1  A  pair  of  Guericke's  "Magdeburg  hemispheres"  were  on  exhibi- 
tion at  the  Cohimbian  Exposition.  As  to  the  fate  of  Guericke's 
original  air-pump,  consult  G.  Berthold  in  Wiedem.  Annalen,  Neue 
Folge,  54,  1895,  pp.  724-726.  Guericke  is  said  to  have  spent  20,000 
thaler  for  apparatus. 

2  The  Works  of  the  Honourable  Bohert  Boyle,  in  five  volumes,  to  which 
is  prefixed  the  Life  of  the  Author,  edited  by  Thomas  Birch,  London, 
1743,  Vol.  L,  p.  6  (of  biography). 


MECHANICS  71 

ment  was  at  hand.  At  this  time  he  became  converted  to 
religion,  and  many  of  his  later  writings  are  on  theology. 
On  his  return  home,  in  1644,  a  youth  of  eighteen,  he  found 
the  country  in  great  confusion  5  nevertheless  he  received  a 
strong  impetus  for  scientific  research  from  the  meetings  in 
London,  in  1645,  of  a  philosophical  society,  —  the  Invisible 
College,  as  he  called  it,  —  which  after  the  Eestoration  was 
incorporated  as  the  Eoyal  Society.  In  1654  he  settled  at 
Oxford,  where  he  erected  a  laboratory,  kept  several  operators 
at  work,  and  engaged  Eobert  Hooke  as  his  chemical  assistant.^ 
After  reading  of  Guericke's  air-pump,  he  let  Hooke  make  a 
less  clumsy  pump,  which  was  completed  in  1659.  As  early  as 
1660  Boyle  published  his  New  Experiments  .  .  .  touching  the 
Spring  of  the  Air. 

He  left  Oxford  for  London  in  1668.  For  forty  years  he 
was  in  feeble  health.  His  memory  was  so  treacherous  that 
he  was  often  tempted  to  abandon  study,  yet  he  was  a  volumi- 
nous writer,  and  possessed  an  immense  reputation  both  at  home 
and  abroad.  Before  1657  he  purposely  refrained  from  "  seri- 
ously and  orderly  "  reading  the  works  of  Gassendi,  Descartes, 
or  Francis  Bacon,  "that  I  might  not  be  prepossessed  with  any 
theory  or  principles  till  I  spent  some  time  in  trying  what 
things  themselves  would  incline  me  to  think."  ^ 

Boyle  placed  the  barometer  in  the  receiver  of  the  air-pump 
and  observed  the  ebullition  of  heated  liquids  and  the  freezing 
of  water  on  exhaustion. 

Except  for  an  absurd  criticism  by  a  would-be  physicist,  Boyle 
would  probably  never  have  discovered  the  law  bearing  his 
name.  Franciscus  Linus,  professor  at  Llittich  in  the  Nether- 
lands, had  read  Boyle's  New  Experiments,  and  declared  that 


1  See  article  "Boyle,  Robert,"  in  Die.  of  Nat.  Biog. 

2  Works,  Vol.  I.,  p.  194. 


72  A   HISTORY    OF   PHYSIOS 

the  air  is  very  insufficient  to  perform  such  great  matters  as 
the  counterpoising  of  a  mercurial  cylinder  of  29  inches ;  he 
claimed  to  have  found  that  the  mercury  hangs  by  invisible 
threads  {funiculi)  from  the  upper  end  of  the  tube,  and  to  have 
felt  them  when  he  closed  the  upper  end  of  the  tube  with 
his  finger.  This  criticism  incited  Boyle  to  renewed  research. 
"We  shall  now  endeavour  to  manifest  by  experiments  pur- 
posely made,  that  the  spring  of  the  air  is  capable  of  doing  far 
more  than  it  is  necessary  for  us  to  ascribe  to  it,  to  solve  the 
phsenomena  of  the  Torricellian  experiment."^  "We  took  then 
a  long  glass  tube,  which  by  a  dexterous  hand  and  the  help  of  a 
lamp  was  in  such  a  manner  crooked  at  the  bottom,  that  the 
part  turned  up  was  almost  parallel  to  the  rest  of  the  tube  and, 
the  orifice  of  this  shorter  leg  .  .  .  being  hermetically  sealed, 
the  length  of  it  was  divided  into  inches  (each  of  which  was 
divided  into  eight  parts)  by  a  straight  list  of  paper,  which 
containing  those  divisions,  was  carefully  pasted  all  along  it." 
A  similar  strip  of  paper  was  pasted  on  the  longer  leg.  Then 
"  as  much  quicksilver  as  served  to  fill  the  arch  or  bended  part 
of  the  siphon  "  was  poured  in  so  as  to  be  at  the  same  height 
in  both  legs.  "  This  done,  we  began  pouring  quicksilver  into 
the  longer  leg  .  .  .  till  the  air  in  the  shorter  leg  was  by  con- 
densation reduced  to  take  up  but  half  the  space  it  possessed 
...  we  cast  our  eyes  upon  the  longer  leg  of  the  glass  .  .  . 
and  we  observed,  not  without  delight  and  satisfaction,  that 
the  quicksilver  in  that  longer  part  of  the  tube  was  29  inches 
higher  than  the  other."  This  tube  was  broken  by  accident, 
and  a  new  one,  about  eight  feet  long,  was  prepared.  It  was 
too  long  to  be  used  in  his  chamber,  so  he  took  it  on  "  a  pair 
of  stairs"  and  suspended  it  by  strings  so  "that  it  did  scarce 
touch  the  box"  placed  underneath.     Pressures  less  than  one 

1  See  "  Defence  against  Linus,"  1662,  Worlcs^  Vol.  I.,  p.  100. 


MECHANICS  73 

atmosphere  were  also  obtained.  Altogether  he  subjected  the 
enclosed  air  to  pressures  varying  from  If  inches  of  mercury 
to  117j^  inches,  passing  from  one  extreme  to  the  other  in 
about  forty  steps,  and  every  time  comparing  the  observed 
pressures  with  what  they  should  be  "  according  to  the  hypothe- 
sis that  supposes  the  pressures  and  expansions  to  be  in  recipro- 
cal proportion."  The  observed  and  theoretical  values  agree 
fairly  well. 

In  1666  Boyle  published  his  Hydrostatical  Paradoxes,  in 
which  he  takes  pains  to  refute  the  old  doctrine  that  a 
light  liquid  can  exert  no  pressure  against  a  heavier  liquid. 
That  such  refutations  seemed  necessary  at  so  late  a  date 
indicates  the  slow  assimilation  of  correct  ideas  on  fluid 
pressure. 

"Boyle's  Law"  was  rediscovered  independently,  fourteen 
years  after  Boyle's  publication  of  it,  by  the  prominent  French 
physicist,  Edme  Mariotte  (1620-1684).  In  France  it  is  always 
called  "Mariotte's  Law."^  Mariotte  published  it  in  his  trea- 
tise, Siir  la  nature  de  Vair,  1676.  He  says,  "  We  employed  a 
tube  of  40  inches,  which  I  filled  with  mercury  up  to  27^  inches, 
121  inches  of  air  being  left,  which,  being  plunged  1  inch  into 
a  vessel  of  mercury,  leaving  39  inches  above,  contained  14 
inches  of  mercury  and  2^  inches  of  air  expanded  to  double  its 
volume."  By  repeated  experimentation  "  it  became  sufficiently 
evident  that  one  may  take  it  as  a  certain  rule  or  law  of  nature 
that  air  condenses  in  proportion  to  the  weight  by  which  it  is 
loaded."  He  had  a  clearer  realization  of  the  importance  of 
this  law  than  had  Boyle. 

1  Makie,  in  his  large  Histoire  des  Sciences  Math,  et  Physi.,  Vol.  IV., 
1884,  pp.  239-242,  gives  an  account  of  Boyle  without  mentioning  the  law 
in  that  connection.  Mariotte  is  represented  as  the  sole  discoverer  (p.  176). 
The  same  course  is  followed  by  A,  Libes  in  hisJEstoij-e  Philosophique  des 
Progres  de  la  Physique,  Paris,  1810,  Vol.  II.,  pp.  134-140,  195. 


74  A   HISTORY   OF   PHYSICS 

To  Mariotte  is  attributed  the  instauration  of  experimental 
physics  in  France.  As  Boyle  was  prominent  in  the  organiza- 
tion of  the  E-oyal  Society  of  London,  so  Mariotte  was  one  of 
the  first  and  leading  members  of  the  Academic  des  Sciences, 
founded  in  1666.  By  carefully  measuring  the  height  of  the 
mercury  column  in  a  deep  cellar,  and  then  at  the  newly  built 
astronomical  observatory,  located  on  high  ground  in  Paris, 
he  obtained  an  approximate  formula  for  estimating  height 
by  the  barometer.  He  wrote  an  important  article  on  per- 
cussion. 

In  1674  Denis  Papin  described  an  air-pump  in  which  the 
flask-like  receptacle  with  a  stop-cock,  such  as  had  been  em- 
ployed by  Guericke  and  Boyle,  was  replaced  by  a  plate  and 
bell  glass.  The  credit  for  this  improvement  is  usually  given 
to  Papin,  but  he  himself  ascribes  it  to  Huygens,  who  is  now 
known  to  have  made  this  desirable  innovation  in  1661.-^ 
Papin  Avas  a  pupil  and  assistant  to  Huygens. 

In  the  study  of  falling  bodies  ani  the  motions  of  projectiles 
the  resistance  of  the  air  has  always  complicated  the  phenomena, 
has  usually  perplexed  the  investigators,  and  has  often  supplied 
critics  with  all  sorts  of  objections.  Galileo  made  allowances 
for  the  resistance  of  the  air.  About  1670  Mariotte  concluded 
from  experiments  at  the  Paris  observatory  that  the  resist- 
ance to  falling  bodies  is  proportional  to  the  square  of  the 
time.  Newton  inclined  to  the  same  conclusion,  while  La  Hire 
favoured  the  cube  of  the  time. 

In  1679  ISTewton  remarked  "that  a  falling  body  ought  by 
reason  of  the  earth's  diurnall  motion  to  advance  eastward  and 
not  fall  to  the  west  as  the  vulgar  opinion  is."  We  may  here 
state,  parenthetically,  that  in  Prance  Mersenne  and  Petit  fired 
bullets  vertically  upward,  expecting  them  to  strike  the  ground 

IE.  Gerland,  Wiedemann's  Annalen,  Vol.  II.,  1878,  p. 


MECHANICS  75 

far  to  the  westward.^  But  the  bullets  could  not  be  found! 
Descartes,  the  French  oracle  of  the  time,  was  consulted,  and 
he  seriously  replied  that  the  bullets  had  received  such  intense 
velocity  that  they  lost  their  weight  and  flew  away  from  the  earth. 

]N'ewton's  prediction  applied,  not  to  a  body  rising  and  then 
falling,  but  to  one  falling  from  rest.  The  experiment  was 
tried  by  his  contemporary,  Robert  Hooke,  who  reported  to  the 
Eoyal  Society  that  he  "had  found  the  ball  in  every  one  of  the 
said  experiments  fall  to  the  southeast  of  the  perpendicular 
point  found  by  the  same  ball  hanging  perpendicular."  The 
experiments  were  made  in  the  open  air,  and  the  results  were 
somewhat  discordant.  "But,"  says  Hooke,  ''within  doors  it 
succeeded  also."^  The  strange  southerly  component  of  the 
deviation  was  probably  ascribed  to  errors  of  observation,  but 
careful  experiments  made  by  G.  B.  Guglielmini  in  1791  from  a 
tower  at  Bologna,  by  J.  F.  Benzeyiberg  in  1802  from  St. 
MichaePs  tower  in  Hamburg,  and  by  F.  Reich  in  1831  down 
a  mine  shaft  at  Freiberg  in  Saxony,  all  showed,  in  addition  to 
the  predicted  easterly  deviation,  also  a  small  southerly  dis- 
placement. For  this  no  satisfactory  explanation  has  yet  been 
given.^ 

In  projectiles  the  actual  path,  as  represented  by  the  ballistic 
curve,  deviates  considerably  from  Galileo's  parabolas.  It  is 
mathematically  almost  unmanageable.  The  path  appears  in 
the  northern  hemisphere  to  be  slightly  bent  to  the  right,  owing 

1  If  disturbances  due  to  the  atmosphere  are  negligible,  then  the  bullets 
should  fall  a  small  distance  to  the  west.  See  W.  Ferrel,  A  Popular 
Treatise  on  the  Winds,  New  York,  1889,  p.  88. 

'^  Birch,  History  of  the  Boyal  Society,  London,  1757,  Vol.  III.,  p.  519, 
Vol.  IV.,  p.  5.  See  also  Ball,  An  Essay  on  Newton'' s  "  Principia,''''  pp. 
146,  149,  150. 

3  RosENBERGER,  Part  III.,  pp.  96,  97,  432-437.  J.  F.  W.  Gronau,  His- 
torische  Entwicklung  der  Lehre  vom  Luftwiderstande,  Danzig,  1868,  pp. 
1-28. 


76  A   HISTORY   OF   PHYSICS 

to  the  rotation  of  the  earth.  That  the  resistance  of  the  ait 
complicates  the  path  of  a  rotating  sphere  is  known  to  every 
base-ball  or  tennis  player.^ 

LIGHT 

The  law  of  refraction  was  discovered  by  Willebrord  Snell 
(1591-1626),  professor  of  mechanics  at  Leyden.  He  never 
published  his  discovery,  but  both  Huygens  and  Isaak  Voss 
claim  to  have  examined  Snell's  manuscript.  He  stated  the 
law  in  inconvenient  form  as  follows :  For  the  same  media  the 
ratio  of  the  cosecants  of  the  angle  of  incidence  and  of  refrac- 
tion retains  always  the  same  value.  As  the  cosecants  vary 
inversely  as  the  sines,  the  equivalence  of  this  to  the  modern 
form  becomes  evident.  As  far  as  known,  Snell  did  not  attempt 
a  theoretical  deduction  of  the  law,  but  he  verified  it  experi- 
mentally. The  law  of  sines,  as  found  in  modern  books,  was 
given  by  Descartes  in  his  La  Dioptrique,  1637.  He  does  not 
mention  Snell,  and  probably  discovered  the  law  independently.^ 
Descartes  made  no  experiments,  but  deduced  the  law  theoreti- 
cally from  the  following  assumptions  :  (1)  the  velocity  of  light 
is  greater  in  a  denser  medium   (now  known  to  be  wrong); 

1  For  the  effect  of  the  earth's  rotation,  consult  Eerrel,  op.  cit..,  p.  86  ; 
PoissoN,  Journ.  Ecole  Pohjterhnique,  XXVI.,  1838.  In  ibidem,  XXVII. , 
1839,  Poissox  considers  the  effect  of  their  rotation  in  the  air.  See  also 
Magnus,  Poggendorff  Annalen.,  LXXXVIIL,  1853,  p.  1. 

2  Various  opinions  have  been  held  on  this  point.  Heller,  Vol.  II., 
pp.  65,  78,  argues  in  favour  of  the  independent  discovery  ;  Poggendorff, 
p.  312,  and  Rosenberger,  Part  II.,  p.  113,  incline  to  the  opinion  that 
Descartes  plagiarized  from  Snell.  Arago,  on  the  other  hand,  declared 
Descartes  the  sole  discoverer.  See  "Fresnel"  in  Arago's  Biographies^ 
2d  series,  Boston,  1859,  pp.  187,  188.  The  question  is  minutely  discussed 
by  P.  Kramer,  Zeitsch.  f.  Math.  u.  Phys.,  Vol.  27,  1882,  Supplement, 
p.  235,  and,  after  the  discovery  of  some  new  documents,  again  by  D.  J. 
Korteweg,  Bevue  de  Metaphysique  et  de  Morale,  July,  1896,  pp.  489-501. 


LIGHT  77 

(2)  for  the  same  media  these  velocities  have  the  same  ratio  for 
all  angles  of  incidence ;  (3)  the  velocity  component  parallel  to 
the  refracting  surface  remains  unchanged  during  refraction 
(now  known  to  be  wrong).  The  improbability  of  the  correct- 
ness of  these  assumptions  brought  about  attacks  upon  the 
demonstration  from  the  mathematician  Permat  and  others. 
Fermat  deduced  the  law  from  the  assumption  that  light 
travels  from  a  point  in  one  medium  to  a  point  in  another 
medium  in  the  least  time,  and  that  the  velocity  is  less  in  the 
denser  medium.^ 

A  great  achievement  of  the  seventeenth  century  was  the 
discovery  of  the  gradual  propagation  of  light.  Previously  its 
speed  was  usually  supposed  to  be  infinite.  The  first  attempt 
to  measure  the  velocity  was  made  by  Galileo.'^  He  ascertained 
the  time  required  for  a  person  A  to  signal  with  a  lantern  to  B 
and  receive  back  a  signal  from  B.  This  was  tried  in  night- 
time, when  the  two  observers  were  stationed  close  together, 
and  also  when  they  were  nearly  a  mile  apart.  If  a  difference 
in  time  could  be  detected,  then  light  would  travel  with  finite 
velocity.  Galileo  was  not  able  to  settle  the  question  from  his 
experiments.  But  he  made  a  suggestion  on  a  wholly  different 
problem  which  accidentally  led  another  investigator  to  success. 
He  remarked  that  the  frequent  disappearance  of  Jupiter's 
satellites  behind  the  planet  might  be  made  to  serve  in  longitude 
determinations.  About  1642  the  Italian  astronomer,  Giovanni 
Domenico  Cassini,  one  of  a  number  of  great  scientists  called 
to  Paris  by  Louis  XIY.,  undertook  a  prolonged  study  of  the 
Jovian  system.  About  thirty  years  later  a  young  Dane,  Olaf 
Rbmer  (1644-1710),  was  induced  to  settle  in  Paris.  He  was  a 
native  of  Aarhus  and  had  studied  at  Copenhagen.     At  Paris  he 


1  RosENBERGER,  Part  II.,  p.  114. 

2  Ostwald's  Klassiker,  No.  11,  pp.  39,  40. 


78  A  HISTORY   OF   PHYSICS 

observed,  together  with  Jean  Picard,  the  eclipses  of  Jupiter's 
moons.  It  was  noticed  that  the  times  of  revolution  of  these 
moons  in  their  orbits  were  not  the  same  at  all  periods  of  the 
year,  and  were  greater  than  the  average  when  the  ap^Darent 
size  of  Jupiter  was  diminishing.  Considering  it  in  the  highest 
degree  improbable  that  the  actual  motions  should  be  affected 
with  any  inequality  of  this  sort,  E-omer  became  convinced 
that  the  observed  irregularities  must  be  explained  on  the  sup- 
position that  the  velocity  of  light  is  finite.  In  September, 
1676,  Romer  stated  to  the  French  Academy  of  Sciences  that 
in  November  next  the  eclipses  of  the  first  satellite  would  be 
about  ten  minutes  later  than,  the  time  gotten  from  computa- 
tions based  on  the  observations  of  the  preceding  August,  and 
that  the  discrepancy  could  be  explained  by  assuming  that  it 
took  time  for  light  to  come  from  Jupiter  to  the  earth.  On 
November  9  an  eclipse  took  place  at  5  h.  35  m.  45  s.,  while 
by  computation  it  should  have  been  at  5  h.  25  m.  45  s.  On 
November  22  he  explained  his  theory  to  the  Academy  more 
fully,  and  said  that  it  required  light  22  minutes  to  cross  the 
earth's  orbit.  (The  more  correct  value  is  now  known  to  be 
16  minutes  and  36  seconds.)  The  Academy  did  not  at  once 
accept  Romer's  theory.  Picard  favoured  it,  but  Cassini  did 
not.  E-omer  had  based  his  computation  on  the  first  satellite, 
and  he  frankly  stated  that  similar  calculations  from  obser- 
vations on  the  three  other  moons  would  not  have  led  to  suc- 
cess. In  Cassini's  mind  this  fact  operated  strongly  against  the 
acceptance  of  Eomer's  explanation.  Regarding  the  behaviour 
of  these  three  bodies,  Eomer  could  only  say  that  "  they  have 
irregularities  not  yet  determined."  In  1680  Cassini  published 
improved  ephemerides  of  Jupiter's  moons,  but  made  no  men- 
tion of  Eomer's  hypothesis. 

The  young  Dane's  fame  increased  to  such  an  extent  that 
he  was  made  tutor  to  the  Dauphin,  and  in  1681  Christian  Y. 


LIGHT  79 

called  him  to  Denmark  as  astronomer-royal.  After  Bomer's 
return  to  his  native  country  confidence  in  his  theory  waned 
at  Paris.  It  is  not  known  how  much  more  he  worked  on  the 
problem,  and  whether  he  removed  the  objection  arising  in 
connection  with  the  other  moons.  He  left  behind  many  as- 
tronomical observations,  nearly  all  of  which  were  destroyed 
by  the  fire  which  devastated  the  town  of  Copenhagen  in  1728.^ 
In  England  Romer's  theory  was  enthusiastically  supported 
by  Edmund  Halley  and  verified  in  an  unexpected  manner 
by  James  Bradley  (1693-1762),  then  Savilian  Professor  of 
Astronomy  at  Oxford.  While  endeavouring  to  determine  the 
parallax  of  a  star,  he  was  surprised  to  find  that  its  displace- 
ment was  not  at  all  as  he  expected  it  to  be.  He  had  almost 
despaired  of  being  able  to  explain  this,  when  an  unexpected 
light  fell  upon  him.  "Accompanying  a  pleasure  party  in  a 
sail  on  the  Thames  one  day  about  September,  1728,  he  noticed 
that  the  wind  seemed  to  shift  each  time  that  the  boat  put 
about,  and  a  question  put  to  the  boatman  brought  the  (to  him) 
significant  reply  that  the  changes  in  direction  of  the  vane  at 
the  top  of  the  mast  were  merely  due  to  changes  in  the  boat's 
course,  the  wind  remaining  steady  throughout.  This  was  the 
clue  he  needed.  He  divined  at  once  that  the  progressive 
transmission  of  light,  combined  with  the  advance  of  the  earth 
in  its  orbit,  must  cause  an  annual  shifting  of  the  direction 
in  which  the  heavenly  bodies  are  seen  by  an  amount  depend- 
ing upon  the  ratio  of  the  two  velocities."  ^  Erom  the  value 
of  this  "  aberration  of  light "  Bradley  estimated  that  solar  rays 
reach  the  earth  in  8  m.  13  s,  This  value  was  more  nearly 
correct  than  Eomer's  11  m.,  determined  half  a  century  earlier. 

1  We  have  used  an  article  on  Olaf  Romerby  Alex.  Wernicke,  Zeitsch. 
f.  Math.  u.  PhysiTc,  Vol.  25, 1880,  Hist.  Abtheil.,  pp.  1-6  ;  also  W.  Dobekck, 
in  Nature,  Vol.  17,  1877,  p.  105. 

2  "Bradley,  James,"  in  Die.  Nat.  Biog. 


80  A   HISTORY   OF   PHYSICS 

Thus  Bradley  verified  E,omer's  theory,  and  the  gradual  propa 
gation  of  light  came  to  be  accepted  as  an  established  fact. 

At  a  meeting  of  the  French  Academy  of  Sciences,  in  1678, 
in  the  presence  of  Eomer,  Cassini,  and  others,  a  remarkable 
paper  on  the  theory  of  light  was  presented  by  Christian  Huy- 
gens  (1629-1695).  He  was  a  native  of  The  Hague,  and  had 
studied  at  the  university  in  Leyden.  The  perusal  of  some  of 
his  earliest  mathematical  theorems  led  Descartes  to  predict  his 
future  greatness.  He  was  induced  by  Louis  XIV.  to  settle  in 
Paris,  where  he  remained  from  1666  to  1681.  Like  his  great 
contemporaries,  Newton  and  Leibniz,  Huygens  never  married. 
Huygens's  Traite  de  la  lumih^e,  of  1678,  referred  to  above, 
was  printed  in  1690.^    It  is  the  earliest  important  attempt  at 

an  exposition  of  the  wave  theory 
of  light.  Before  Huygens,  a 
rough  outline  of  such  a  theory 
had  been  given  in  1665,  by 
Eobert  Hooke.  Huygens  de- 
velops the  important  principle, 
known  by  his  name,  relating 
to  the  propagation  of  waves. 
Around  each  particle  of  a  vi- 
brating medium  as  a  centre,  a 
wave  is  formed.  Thus,  if,  in  Fig.  11,  DCF  is  a  spherical  wave 
which  starts  from  ^  as  centre,  then  a  particle  B  within  this 
sphere  will  be  the  centre  of  a  special  wave  KCL,  touching 
DCF  at  C.  In  the  same  way  every  other  particle  inside  the 
sphere  DCL  forms  a  wave  of  its  own.  All  these  innumerable 
feeble  wavelets  are  spheres,  each  touching  DCL  at  one  point, 
and   contributing   to   its    formation.     Huygens    assumes   the 

1  Eeprinted  in  German  translation  in  Ostwald''s  Klassiker,  No.  20. 
Consult  also  (Euvres  Completes  de  Christiadn  Huygens,  publicees  par  la 
Societe  Hollandaise  des  Sciences,  La  Haye,  1888-1896. 


LIGHT  81 

existence  of  an  all-pervading  ether,  and  explains  reflection  and 
refraction  of  light  by  the  wave  theory  in  the  manner  current  in 
modern  texts.  Atmospheric  refraction,  and  the  marvels  of 
double  refraction  in  the  Iceland  spar,  are  dwelt  upon.  This 
division  of  a  ray  was  first  observed  in  Iceland  spar  in  1669, 
by  Erasmus  BartJwUnus,  of  Copenhagen.  Huygens  gave  the 
method  of  constructing  the  path  of  the  ordinary  and  extraor- 
dinary ray,  and  observed  that  these  rays  were  polarized.  He 
assumed  the  vibrations  in  the  ether  to  be  longitudinal,  as  in 
sound,  and  was,  therefore,  not  able  to  explain  the  strange 
phenomenon  of  polarization.  ISTor  could  he,  by  his  theory, 
explain  the  origin  of  colours.  He  endeavoured  to  deduce  from 
the  wave  theory  the  fact  that  light  travels  rectilinearly  in  a 
homogeneous  medium.  His  argument  was  not  conclusive. 
The  main  reason  why  Newton  rejected  the  undulatory  theory 
was  its  apparent  inability  to  explain  satisfactorily  why  light 
travels  in  straight  lines.  Newton  threw  the  weight  of  his 
great  authority  on  the  side  of  the  emission  theory,  and  for 
over  a  century  Huygens's  ideas  were  laid  aside  and  neglected. 

Notwithstanding  Newton's  advocacy  of  a  theory  now  known 
to  be  erroneous,  his  researches  on  light  are  of  the  great- 
est importance,  and  give  evidence  of  extraordinary  powers. 
Newton's  first  observations  are  on  coronas,  and  date  back  to 
his  student  days  in  1664.  Later  come  his  experiments  on 
dispersion.  "In  the  year  1666  (at  which  time  I  applied 
myself  to  the  grinding  of  optick  glasses  of  other  figures  than 
spherical)  I  procured  me  a  triangular  glass  prism  to  try  there- 
with the  celebrated  phenomena  of  colours." 

The  formation  of  colours  from  white  light  had  been  observed 
long  ago.  Seneca  (2-66  a.d.)  spoke  of  the  identity  of  rain- 
bow colours  and  those  formed  by  the  edges  of  a  piece  of  glass. 
The  breaking  up  or  condensation  of  white  light  into  colours  was 
discussed  by  Marcus  Marci,  professor  of  medicine  at  Prague 


82  A    HISTORY   OF   PHYSICS 

(1648),  by  Grimaldi,  Descartes,  Hooke,  and  others.^  Isaaa 
Barrow,  Newton's  teacher  at  Cambridge,  held  a  theory  re- 
sembling one  of  Marcus  Marci,  that  red  was  strongly  con- 
densed light,  that  violet  was  strongly  rarefied  light.  It 
remained  for  Newton  to  remove  the  cobwebs  and  point  out 
the  cause  of  dispersion. 

In  a  darkened  room  he  made  a  small  circular  opening  in  the 
shutter  and  placed  the  prism  inside,  near  the  hole,  so  that 
the  light  was  refracted  to  the  opposite  wall.  "  Comparing  the 
length  of  this  coloured  spectrum  with  its  breadth,  I  found  it 
about  five  times  greater  —  a  disproportion  so  extravagant,  that 
it  excited  me  to  a  more  than  ordinary  curiosity  of  examining 
from  whence  it  might  proceed."^ 

Before  reaching  the  right  explanation  he  advanced  several 
hypotheses,  only  to  find  that  each  was  disproved  by  the  facts. 
One  of  these  guesses  is  of  particular  interest  to  the  college 
students  of  to-day,  as  it  shows  that  Newton's  profound  mind 
had  dwelt  upon  a  subject  prominent  in  modern  athletics, 
namely,  the  subject  of  "curved  pitching."  Surely  the  modern 
student  would  find  it  hard  to  guess  what  possible  relation 
there  might  be  supposed  to  exist  between  the  performance  of 
a  twirler  on  the  diamond  and  optical  theories.  Here  is  what 
Newton  said:  "Then  I  began  to  suspect,  whether  the  rays, 
after  their  trajection  through  the  prism,  did  not  move  in  curve 
lines  and  according  to  their  more  or  less  curvity  tend  to  divers 
pa,rts  of  the  wall.     And  it  increased  my  suspicion,  when  I 


1  Pater  Trigautius,  in  the  description  of  his  mission  to  China,  narrates 
that  prisms  were  highly  valued  for  their  colour  effects,  and  were  usually 
owned  only  by  persons  in  high  authority,  and  that  a  single  piece  sold  for 
500  pieces  of  gold.  Priestley,  Gesch.  d.  Optik,  trans,  by  G.  S.  Klugel, 
Leipzig,  1776,  p.  132. 

^  Phil.  Trans. ^  Abr.,  Vol.  I.,  p.  128.  Newton  sent  this  article  to 
the  Royal  Society  in  1672. 


LIGHT  83 

remembered  that  I  had  often  seen  a  tennis  ball  struck  with  an 
oblique  racket,  describe  such  a  curve  line.  Eor,  a  circular  as 
well  as  a  progressive  motion  being  communicated  to  it  by  that 
stroke,  its  parts  on  that  side,  where  the  motions  conspire,  must 
press  and  beat  the  contiguous  air  more  violently  than  on  the 
other,  and  there  excite  a  reluctancy  and  reaction  of  the  air 
proportionably  greater.  And  for  the  same  reason,  if  the  rays 
of  light  should  possibly  be  globular  bodies,  and  by  their 
oblique  passage  out  of  one  medium  into  another,  acquire  a  cir- 
culating motion,  they  ought  to  feel  the  greater  resistance  from 
the  ambient  aether,  on  that  side,  where  the  motions  conspire, 
and  thence  be  continually  bowed  to  the  other.  But  notwith- 
standing this  plausible  ground  of  suspicion,  when  I  came  to 
examine  it,  I  could  observe  no  such  curvity  in  them.  And 
besides  (which  was  enough  for  my  purpose)  I  observed,  that 
the  difference  betwixt  the  length  of  the  image,  and  the  diameter 
of  the  hole,  through  which  the  light  was  transmitted,  was  pro- 
portionable to  their  distance. 

"  The  gradual  removal  of  these  suspicions  at  length  led  me 
to  the  exjjerimentum  crucis,  which  was  this :  I  took  two  boards, 
and  placed  one  of  them  close  behind  the  prism  at  the  icindow,  so 
that  the  light  might  pass  through  a  small  hole,  made  in  it  for 
the  purpose,  and  fall  on  the  other  board,  which  I  placed  at 
about  twelve  feet  distance,  having  first  made  a  small  hole  in  it 
also,  for  some  of  that  incident  light  to  pass  through.  Then, 
I  placed  another  prism  behind  the  second  board."  On  turning 
the  first  prism  about  its  axis,  the  image  which  fell  on  the 
second  board  was  made  to  move  up  and  down  upon  that  board, 
so  that  all  its  parts  could  successively  pass  through  the  hole  in 
that  board,  and  fall  upon  the  prism  behind  it.  The  places 
where  the  light  fell  against  the  wall  were  noted.  It  was  seen 
that  the  blue  light,  which  was  most  refracted  in  the  first  prism, 
was  also  most  refracted  in  the  second  prism,  the  red  being 


84 


A    HISTORY    OF   PHYSICS 


least  refracted  in  both  prisms.  "  And  so  the  true  cause  of  the 
length  of  that  image  was  detected  to  be  no  other  than  that 
light  is  not  similar  or  homogeneal,  but  consists  of  difform  rays, 
some  of  lohicli  are  more  refrangible  than  others."  ^  (See  Fig.  12.) 
When  Newton  made  these  experiments,  he  was  interested  in 
the  improvement  of  the  refracting  telescope.  The  deficiencies 
noticed  in  that  instrument  had  always  been  attributed  to 
spherical  aberration  and  the  attempt  was  being  made  so  to 


Fig.  12. 


alter  the  spherical  form  of  lenses  as  to  give  clear  images. 
Newton  satisfied  himself  that,  besides  spherical  aberration, 
there  was  another  source  of  trouble,  namely,  chromatic  aberra- 
tion. "  The  confused  vision  of  objects  seen  through  refracting 
bodies  by  heterogeneal  light  arises  from  the  different  refran- 
gibility  of  several  sorts  of  rays."  (Opticks,  Book  I.,  Prop.  V.) 
Could  this  evil  be  removed  ?  Probably,  if  different  substances 
possessed  different  dispersive  powers.  So  Newton  contrived 
an  experiment.  In  a  prismatic  vessel  filled  with  water  (prob- 
ably it  was  impregnated  with  saccharum  saturni  —  sugar  of 
lead),^  he  placed  a  glass  prism  and  examined  rays  passing 
through.    Prom  his  tests  he  thought  he  could  conclude  that  re- 

^  Phil.   Trans..,  Abr.,  Yol.  I.,  p.  130.      These  experiments  are  also 
described  in  Newton's  Opticks,  Book  I,,  Props.  I.-V. 
^  Opticks,  p.  51 ;  "Newton,  Isaac,"  in  Die.  Nat.  Biog. 


LIGHT  85 

fraction  must  always  be  accompanied  by  dispersion.  Achro- 
matic lenses  seemed  to  him  an  impossibility.  Evidently, 
here  Newton  did  not  exercise  his  usual  caution.  He  happened 
to  have  used  a  prism  of  glass  and  one  of  water  of  equal  disper- 
sive powers.  Other  liquids  than  his  impregnated  water  would 
have  given  different  results.  From  very  limited  experimental 
evidence  he  drew  a  broad  inference,  to  which  he  adhered  with 
marvellous  tenacity,  but  which  later  experimenters  have  found 
to  be  erroneous. 

Despairing  of  the  possibility  of  producing  achromatic  re- 
fractors, he  entered  upon  the  design  of  reflectors.  At  that 
time,  the  reflecting  telescope  had  been  the  subject  of  con- 
siderable attention.  Niccolo  Zucclii  (1586-1670),  a  Jesuit  in 
Eome,  is  considered  the  inventor  of  it.  Another  Jesuit, 
Marin  Mersenne,  in  France,  suggested  a  different  type  of 
reflector,  as  did  also  the  Scotch  mathematician  and  astrono- 
mer, James  Gregory  (1638-1675).  But  they  did  not  carry  out 
their  designs.  Newton  constructed  his  first  reflecting  tele- 
scope in  1668.  It  was  six  inches  long,  had  a  diameter  of 
one  inch,  and  magnified  30  to  40  times.  Later  he  made  a 
larger  instrument,  which  he  presented  to  the  Eoyal  Society  in 
1672,  with  the  inscription,  "Invented  by  Sir  Isaac  Newton 
and  made  with  his  own  hands,  1671."  It  was  shown  to 
the  king  and  was  examined  by  Eobert  Hooke,  Christopher 
Wren,  and  others.  It  was  greatly  admired,  and  a  description 
of  it  was  sent  to  Huygens  in  Paris. ^  The  telescope  is  pre- 
served in  the  library  of  the  Eoyal  Society. 

Newton's  discoveries  were  well  received  by  the  Eoyal 
Society,  but  as  soon  as  they  were  published  in  the  Philo- 
sophical  Transactions  he  was  opposed   by  several  critics,  — 


1 H.  Servus,  Gesch.  d.  Fernrohrs,  1886,  pp.  121-132  ;  D.  Brewsteb, 
Life  of  Sir  Isaac  Newtoji,  New  York,  1831,  p.  40. 


86  A   HISTORY    OF   PHYSICS 

Linus,  Lucas,  Pardies,  Hooke,  Huygens.  Newton  was  over 
sensitive  to  criticism,  and  in  December  9,  1675,  wrote  to 
Leibniz,  "I  was  so  persecuted  with  discussions  arising  from 
the  publication  of  my  theory  of  light,  that  I  blamed  my  own 
imprudence  for  parting  with  so  substantial  a  blessing  as  my 
quiet,  to  run  after  a  shadow." 

Hooke  upheld  the  undulatory  theory  of  light  as  against 
Newton's  corpuscular  theory.  Newton's  reply  to  Hooke,  as 
well  as  other  papers  communicated  between  1672  and  1676, 
show  that  he  had  carefully  weighed  the  arguments  for  and 
against  each  hypothesis.  We  can  readily  imagine  how  the 
young  scientist  pondered  over  the  two  rival  theories ;  and 
when  he  hesitatingly  rejected  the  wave  theory,  he  little 
dreamed  that  his  views  would  ever  command  such  great 
authority,  and  bias  the  minds  of  physicists  to  such  an  extent 
as  to  delay  for  a  whole  century  the  acceptance  of  the  true 
theory.  Newton  had  experimented  on  colours  formed  by  thin 
plates.^  He  saw  plainly  how  the  phenomena  might  be  ex- 
plained by  the  undulatory  theory.  "  Since  the  vibrations 
which  make  blue  and  violet  are  supposed  shorter  than  those 
which  make  red  and  yellow,  they  must  be  reflected  at  a  less 
thickness  of  the  plate ;  which  is  sufficient  to  explicate  all  the 
ordinary  phenomena  of  those  plates  or  bubbles,  and  also  of 
all  natural  bodies,  whose  parts  are  like  so  many  fragments 
of  such  plates.  These  seem  to  be  the  most  plain,  genuine, 
and  necessary  conditions  of  this  hypothesis ;  and  they  agree 
so  justly  with  my  theory,  that,  if  the  animadversor  think  fit 
to  apply  them,  he  r.eed  not,  on  that  account,  apprehend  a 

1  "Newton's  Rings"  are  explained  in  Newton's  Opticks,  published  in 
1704,  Book  II.,  Obs.  I.  et  seq.  The  colours  of  thin  plates  had  been  ob- 
served by  Boyle  and  Hooke.  The  latter  gave  correct  accounts  of  the 
leading  phenomena  as  exhibited  in  the  coloured  rings  in  soap-bubbles  and 
between  plates  of  glass  compressed  together. 


LIGHT  87 

divorce  from  it;  but  yet,  how  he  will  defend  it  from  other 
difficulties  I  know  not."  ^  In  Kewton's  mind  the  insuperable 
barrier  to  the  acceptance  of  the  wave  theory,  as  it  was  devel- 
oped at  that  time,  was  its  inability  to  explain  the  rectilinear 
path  of  rays.  He  says  :  "  To  me  the  fundamental  supposition 
itself  seems  impossible,  namely,  that  the  waves  or  vibrations 
of  any  fluid  can,  like  the  rays  of  light,  be  propagated  in 
straight  lines,  without  a  continual  and  very  extravagant 
spreading  and  bending  every  way  into  the  quiescent  medium, 
where  they  are  terminated  by  it.  I  mistake  if  there  be  not 
both  experiment  and  demonstration  to  the  contrary."^  If 
light  consisted  of  vibrations,  it  would,  like  sound,  "  bend  into 
the  shadow." 

The  emission  theory,  on  the  other  hand,  offered  an  easy 
explanation.  A  luminous  body  emits  streams  of  minute  parti- 
cles moving  in  straight  lines,  which  cause  vision  by  their 
impact  on  the  retina.  Refraction  was  explained  by  assuming 
that  the  flying  particle  begins  to  be  attracted  towards  the 
refracting  surface  when  it  comes  very  near,  so  that  the  com- 
ponent of  its  velocity  along  the  normal  is  increased.  When 
the  particle  passes  from  a  denser  to  a  rarer  medium,  this  com- 
ponent is  decreased,  while  the  component  velocity  perpendicu- 
lar to  the  normal  remains  unaltered  in  both  cases.  Thus  the 
bending  of  the  ray  is  explained.  As  a  consequence,  the  velocity 
of  the  particle  is  greater  through  the  denser  medium.^  The 
fact  that,  in  a  transparent  substance,  there  exists  both  reflec- 
tion and  refraction  was  very  difficult  to  explain  on  the  emis- 
sion theory.     How  can  a  surface  at  one  time  refract  and  at 

1  Phil.  Trans.,  Abr.,  Vol.  I.,  p.  145 ;  quoted  in  G.  Peacock,  Miscella- 
neous Works  of  the  Late  Thomas  Young ^  Vol.  I,,  pp.  145,  146. 

2  Phil.  Trans.,  Abr.,  Vol.  I.,  p.  146  ;  Miscellaneous  Works  of  Thomas 
Young.,  Vol.  I.,  p.  152. 

8  Newton's  Opticks,  1704,  Book  II.,  Part  III.,  Prop.  X. 


88  A   HISTORY   OF  PHYSICS 

another  time  reflect  an  impinging  particle  ?  To  account  io\ 
this,  Newton  advanced  the  theory  of  "fits"  of  easy  reflection 
and  easy  transmission,  communicated  to  the  particles  by  the 
all-pervading  ether.^  Observe  that  Newton's  emission  theory 
postulates  the  existence,  not  only  of  the  flying  particles  consti- 
tuting light,  but  also  of  an  ether  —  all  the  mechanism  needed 
for  the  wave  theory,  and  more. 

Newton  gave  an  explanation  of  the  rainbow,  the  correct 
outline  of  which  had  been  given  previously  by  Archbishop 
Antonius  de  Domini  in  a  book  published  in  1611,  as  also  by 
Descartes  and  Huygens. 

Newton  experimented  also  on  diffraction  ("  inflection ") 
of  light.  The  discovery  of  this  phenomenon  was  made  by 
Francesco  Maria  Grimaldi  (1618-1663),  professor  of  mathe- 
matics at  the  Jesuit  College  in  Bologna.  It  was  described  in 
his  work,  Physico-mathesis  de  lumine,  1666.  Through  a  small 
hole  Grimaldi  introduced  a  pencil  of  light  into  a  dark  room. 
The  shadow  cast  by  a  rod  held  in  the  cone  of  light  was  allowed 
to  fall  upon  a  white  surface.  To  his  surprise  he  found  the 
shadow  wider  than  the  computed  geometrical  shadow;  more- 
over,  it  was  bordered  by  one,  two,  and  sometimes  three  coloured 
bands.  When  the  light  was  very  strong,  he  saw,  in  addition, 
coloured  bands  inside  the  shadow  itself.  On  replacing  the  rod 
by  an  opaque  plate  with  a  small  hole  in  it,  the  illuminated 
circle  was  found  larger  than  it  should  have  been,  on  the  sup- 
position that  the  rays  travelled  past  the  edges  of  the  hole  in 
exactly  straight  lines.  This  and  other  experiments  established 
the  fact  that  light  bends  very  slightly  around  a  corner.  He 
called  the  new  phenomenon  "  diffraction."  ^ 

1  Opticks,  Book  II.,  Part  III.,  Prop.  XIIT.  ;  see  also  T.  Preston,  77ie 
Theory  of  Light,  2d  ed.,  1895,  p.  19.  Preston  gives  a  good  resum^  of 
the  emission  theory. 

2  RosENBERGER,  Part  II.,  pp.  131,  132. 


HEAT  89 

Grimaldi's  experiments  were  ably  conducted,  but  he  was 
unable  to  contribute  anything  substantial  to  their  theory. 
Newton  repeated  Glrimaldi's  experiments  in  modified  form 
and  endeavoured  to  explain  them  by  the  emission  theory.^ 

It  is  reniarkable  that  Newton  should  have  experimented  so 
much  with  the  solar  spectrum  and  have  failed  to  observe  the 
Fraunhofer  lines.  We  cannot  attribute  this  failure  to  his 
introduction  of  light  through  a  circular  opening,  for  in  some 
cases  (Book  I.,  Prop.  TV.,  p.  49)  he  employed  a  narrow  slit. 
It  cannot  be  ascribed  to  his  placing  the  prism  close  to  the  open- 
ing so  as  to  receive  upon  it  very  divergent  light,  since  in  the 
case  just  referred  to  the  prism  was  at  a  distance  of  10  or 
12  feet  from  the  slit.  The  fact  that  he  received  the  spectrum 
on  paper  would  not  necessarily  debar  him  from  seeing  dark 
lines;  at  any  rate,  he  sometimes  ^'looked  through  the  prism 
upon  the  hole  "  (Book  I.,  Prop.  II.,  Exp.  4,  p.  22).  In  the  ex- 
periment (p.  49),  the  conditions  were  about  the  same  as  those 
under  which  Wollaston  later  saw  a  few  of  the  lines.  Unfortu- 
nately, during  the  very  experiments  in  which  the  discovery 
of  the  lines  would  have  been  easiest,  Newton  was  obliged  to 
rely  on  the  observations  of  an  assistant  with  "  more  critical  '^ 
eyes  than  his  own,^  but  who  was  probably  less  alert  for  unex- 
pected phenomena. 

HEAT 

During  the  seventeenth  century  we  witness  the  early 
development  of  the  thermometer,  a  physical  instrument  which 
has  enjoyed  wider  application  than  almost  any  other.  Mod- 
ern historical  research  concurs  in  ascribing  its  invention  to 

1  See  OpticTcs,  Book  III.,  pp.  113-137. 

2  Book  I.,  Part  II.,  Exp.  7,  p.  92;  see  also  an  article  on  "Newton, 
Wollaston,  and  Fraunhofer  Lines,"  by  Alexander  Johnson  in  Nature^ 
Vol.  26,  1882,  p.  572. 


90  A   HISTORY    OF   PHYSICS 

Galileo}  A  glass  bulb  of  the  size  of  a  lien's  egg,  with  a  long 
stem  of  the  thickness  of  a  straw,  and  dipping  into  water,  which 
was  made  to  rise  part  way  up  the  tube  by  previous  warming 
of  the  bulb,  constituted  Galileo's  first  thermometer.  It  was 
aifected,  of  course,  by  fluctuations  of  atmospheric  pressure  as 
well  as  of  temperature,  and  was  really  a  therm o-baroscope. 
Galileo's  pupil,  Viviani,  gives  1593  as  the  date  of  the  inven- 
tion ;  CastelU,  another  pupil,  says  that  in  1603  he  saw  Galileo 
use  it  in  experimental  lectures.  All  the  early  thermometers 
contained  air,  and  the  stem  was  arbitrarily  graduated.  Being 
affected  by  changes  in  atmospheric  pressure,  Galileo's  air 
thermometer  was  very  imperfect. 

The  first  improvement  was  introduced  by  the  French  physi- 
cian, Jean  Hey,^  who  simply  inverted  Galileo's  instrument,  fill- 
ing the  bulb  Avith  water  and  the  stem  with  air.  Thus,  water 
was  made  the  thermometric  substance.  On  January  1,  1632, 
he  communicated  this  method  to  the  great  intermediary  among 
scientists.  Pater  Mersenne.  As  Key  could  not  bring  himself 
to  close  the  upper  end  of  the  stem,  there  was  constant  danger 
of  errors  from  evaporation  of  the  water.  Schwenter  says  that 
before  1636  artisans  had  succeeded  in  so  choosing  the  relative 
dimensions  of  bulb  and  stem,  that  the  liquid  rose  and  fell  the 
whole  length  of  the  stem  in  course  of  one  year. 


1  E.  WoHLWiLL,  "Zur  Geschichte  der  Erfindung  und  Verbreitung 
des  Thermometers,"  Poggendorfs  Annalen,  Vol.  124,  1865,  pp.  163- 
178 ;  E.  BuRCKHAKDT,  Die  Erfindung  des  Thermometers  und  seine 
Gestaltung  im  17.  Jahrhundert.  Basel,  1867  ;  E.  Gerland,  Das  Ther- 
mometer^ Berlin,  1885.  Of  Gerland' s  publication  we  have  made  extensive 
use.  The  invention  of  the  thermometer  has  been  variously  ascribed  to 
the  famous  mechanic,  Cornelius  Drebbel  of  Holland,  to  the  anatomist, 
Sanctorius  of  Padua,  to  Father  Paul  of  Cracow,  to  the  London  physician, 
Bohert  Fludd,  to  the  German,  Otto  von  Guericke. 

2  G.  Hellmann  in  Himmel  und  Erde,  Vol.  II.,  p.  172 ;  E.  Gerlakd, 
op.  cit.i  p.  10. 


HEAT  91 

To  some  minds  the  rise  and  fall  of  the  thermometer  presented 
an  example  of  perpetual  motion,  and  one  writer  actually  called 
the  instrument  a  "perpetmwi  mobile  showing  degrees  of  heat 
and  cold."  ^ 

A  quarter  of  a  century  after  Rey's  innovation,  the  idea  of 
sealing  the  tube  was  carried  out  by  the  Florentine  academicians, 
probably  on  the  suggestion  of  Grand  Duke  Ferdinand  II.  of 
Tuscany.  The  tube  was  filled  with  spirit  of  wine  and  a 
graduated  scale  was  attached  to  the  stem. 

These  academicians,  not  more  numerous  than  the  muses, 
were  pupils  of  Galileo,  and  made  the  Accademia  del  Gimento 
(academy  of  experiment)  famous.  In  this  small  organization 
the  spirit  of  Galileo  revived  for  a  time  in  Italy ;  but  the  societj 
lasted  only  ten  years,  1657-1667.  What  was  the  cause  of 
this  early  dissolution  ?  According  to  some  writers,^  Leopold  de* 
Medici,  the  brother  of  the  Grand  Duke,  and  with  him  founder 
and  patron  of  the  organization,  was  given  the  cardinal's  hat 
only  on  condition  that  the  Academy  be  broken  up.  According 
to  others^  there  arose  dissensions  among  the  members  them- 
selves. 

Before  the  organization  of  this  academy,  the  Italians  had 
already  done  much  for  meteorology.  Besides  the  invention 
of  the  thermometer  and  barometer,  they  invented  the  rain- 
gauge,  first  used  by  Benedetto  Castelli  in  1639.'*  The  problem 
of  selecting  two  fixed  temperatures  for  the  thermometer  and 
of  subdividing  the  interval  into  a  suitable  number  of  degrees 
was  taken  up  by  the  Accademia  del  Cimento.  Following  the 
example  of  the  philosophers  and  physicians,  they  chose  as 

^  See  E.  WoHLwiLL,  op.  cit.,  p.  169. 
2  PoGGENDOKFF,  p.  351 ;  KosESTBERGER,  Part  II.,  p.  162. 
s  Gerland,  op.  cit. ,  p.  45  ;  also  his  article  in  Wiedemann's  Annalen, 
Vol.  IV.,  p.  604. 

*  G.  Hbllmann,  op.  cit.,  p.  176. 


92  A   HISTORY    OF   PHYSICS 

fixed  points  the  cold  of  winter  and  the  heat  of  summer,  divid 
lug  the  intervening  space  into  80  or  40  equal  spaces.  To  deter- 
mine more  accurately  the  position  of  these  points,  they  defined 
the  one  to  be  the  temperature  of  snow  or  ice  in  the  severest 
frost,  and  the  other  to  be  the  temperature  in  the  bodies  of 
cows  and  deer.  The  melting-point  of  ice  was  found  by  them 
to  be  invariable,  and,  in  their  medical  scale,  to  be  at  13^°.  In 
1829  some  of  the  Florentine  thermometers  were  discovered 
among  old  glass-ware,  and  Libri  actually  found  them  to  read 
13J-°  in  melting  ice.  They  had  been  used  in  Florence  six- 
teen years  in  meteorological  observations,  and  by  reducing  the 
average  temperature  to  one  of  the  modern  scales,  and  compar- 
ing with  modern  observations,  Libri  thought  he  could  draw  the 
inference  that  the  climate  of  Florence  had  remained  unaltered 
during  the  two  hundred  years.^ 

The  fixed  points  chosen  by  the  Florentine  Academy  did  not 
prove  satisfactory,  and  all  sorts  of  improvements  were  sug- 
gested. Dalence  in  1688  adopted  (1)  the  temperature  of  air 
during  freezing,  and  (2)  that  of  melting  butter.  The  final 
adoption  of  the  temperatures  of  melting  ice  and  boiling  water 
was  not  reached  until  the  eighteenth  century,  though  Huygens 
had  recommended  these  as  early  as  1665.^ 

The  Florentine  thermometers  became  famous.  They  were 
introduced  into  England  by  Boyle.  They  reached  France  by 
way  of  Poland.  An  envoy  of  the  Queen  of  Poland  was  pre- 
sented in  1657  by  the  Grand  Duke  with  thermometers  and 
other  instruments.  Her  secretary  forwarded  one  of  the  ther- 
mometers to  the  astronomer  Ismael  BoulUau  in  Paris  and  stated 
that  "  the  Grand  Duke  always  carries  one  in  his  pocket."  ^    The 

1  Libri,  Poggendorff'^s  Annalen,  Vol.  21,  p.  325;  see  aiso  Gerland, 
Das  Thermometer,  p.  45. 

2  E.  Gerland,  Zeitschr.  f.  Instrk.,  Vol.  13,  1893,  p.  390. 
8  Maze,  Comptes  Bendus,  Vol.  121,  1895,  p.  230. 


HEAT  93 

thermometer  was  about  one  decimeter  long  and  contained  alco- 
hol. Boulliaii  himself  constructed  in  1659  a  thermometer  in 
which  mercury  was  used  for  the  first  time  (so  far  as  known) 
as  a  thermometric  substance.  Recently  a  record  of  tempera- 
ture observations  by  Boulliau,  extending  from  May,  1658,  to 
September,  1660,  has  been  found.  Next  to  the  Florentine 
record,  begun  in  1655,  it  is  the  oldest  in  existence.^ 

We  are  surprised  to  find  that  Newton's  immediate  prede- 
cessors had  anticipated  our  modern  theory  of  heat.  Heat  a 
Mode  of  Motion  is  the  title  of  Tyndall's  well-known  work 
(1862),  yet  Descartes,  Amontons,  Boyle,  Francis  Bacon,  Hooke, 
and  Newton  already  looked  upon  heat  as  a  mode  of  motion. 
Of  course,  in  the  seventeenth  century,  this  theory  rested  upon 
somewhat  slender  experimental  evidence,  else  the  doctrine 
could  hardly  have  been  cast  to  the  winds  by  the  eighteenth- 
century  philosophers.  Boyle  experimented  on  the  mechanical 
production  of  heat  and  illustrated  the  heating  due  to  arrested 
motion  by  such  examples  as  the  hammer  driving  a  nail. 

Boyle  observed  also  the  effects  of  atmospheric  pressure  on 
ebullition  and  experimented  with  freezing  mixtures.  Newton, 
in  1701,  made  a  statement  in  the  Philosophical  Transactions 
which  involves  the  hypothesis  that  the  rate  of  cooling  of  a 
body  is  proportional  to  its  excess  of  temperature  over  the  sur- 
rounding medium.^  This  surmise  has  since  been  tested  experi- 
mentally by  Dulong  and  Petit,  and  has  been  shown  to  be  true 
only  within  a  small  range  of  temperature.^ 

1  Maze,  Comptes  Bendus,  Vol.  120,  1895,  p.  732. 

2  Mach,  Princ.  d.  Wdrmelehre,  p.  132. 

»  Ann.  de  chim.  et  de  Phys.  2^,  Vol.  VII.,  1817,  pp.  225,  237. 


94  A   HISTORY   OF  PHYSICS 

ELECTRICITY   AND   MAGNETISM 

The  correction  of  Gilbert's  error  in  asserting  that  magnetic 
declination  "is  constant  at  a  given  place/'  and  the  discovery 
of  the  "  secular  variation  of  the  declination/'  is  usually  attrib- 
uted to  Henry  Gellihrand  (1597-1637),  professor  at  Gresham 
College.  He  pointed  out  that  in  1580,  ''Mr.  Burrows  (a  man 
of  unquestionable  abilities  in  the  mathematiques)  "  found  the 
declination  near  London  to  be  11°  15'  E. ;  that  in  1622  Edmund 
Gunter  found  it  to  be  at  the  same  place  6°  13' ;  that  in  1634  he 
himself  found  it  to  be  not  much  more  than  4°  E.^  This  subject 
received  the  careful  attention  of  Edmund  Halley  (1656-1742), 
who  was  professor  at  Oxford  and  later  astronomer-royal.  He 
endeavoured  to  explain  magnetic  variation  by  assuming  four 
fixed  magnetic  poles.  As  this  did  not  account  for  the  facts,  he 
supposed  that  the  earth  consisted  of  two  concentric  magnetic 
shells  with  poles  differently  placed  and  not  coincident  with 
the  geographic  poles,  the  inner  shell  rotating  slowly.  In 
1698,  William  III.  was  induced  to  send  Halley  upon  a  long 
voyage  on  the  Atlantic  and  Pacific  oceans  for  the  purpose  of 
testing  his  hypothesis.^  He  came  back,  not  with  the  desired 
proof,  but  with  useful  observations  on  "variation."  About  the 
beginning  of  the  eighteenth  century  he  constructed  charts  of 
equal  variation  (declination),  which  became  famous.  One  of 
his  original  isogenic  maps  has  been  found  recently  in  the 
British  Museum.  It  seems  that  he  published  two  totally  dis- 
tinct charts.^ 

^  Consult  Henry  Gellibrand,  A  Discourse  Mathematical  on  the  Vari- 
ation of  the  Magneticall  Needle^  London,  1635.  Reprinted  in  G.  Hell- 
mann's  NeudrucTce,  No.  9,  Berlin,  1897. 

2  Benjamix,  p.  448. 

3  L.  A.  Bauer,  "  Halley 's  Earliest  Equal  variation  Chart,"  Terrestrial 
Magnetism,  Vol.  I.,  1896,  p.  29;  L.  A.  Bauer,  Nature,  May  23,  1895. 


ELECTRICITY    AND    MAGNETISM  95 

Some  interesting  observations  were  recorded  in  the  PMlo 
sophical  Transactions  of  1676  and  1684  regarding  the  magnetic 
effects  of  lightning.  Thus  m  1681,  a  ship  bound  for  Boston 
was  struck  by  lightning.  Observations  of  the  stars  showed 
that  "the  compasses  were  changed;"  "the  north  point  was 
turn'd  clear  south."  The  ship  was  steered  to  Boston  with  the 
compass  reversed.^ 

Phenomena  due  to  electric  attraction  and  repulsion  con- 
tinued to  interest  and  amuse  investigators.  Thus,  Boyle  ob- 
served that  dry  hair  is  easily  electrified  by  friction.  "That 
false  locks  of  hair,  brought  to  a  certain  degree  of  dryness,  will 
be  attracted  by  the  flesh  of  some  persons,  I  had  proof  in  two 
beautiful  ladies  who  wore  them ;  for,  at  some  times,  I  observed 
that  they  could  not  keep  them  from  flying  to  their  cheeks,  and 
from  striking  there,  tho'  neither  of  them  had  occasion  for  or 
did  use  paint."  One  of  the  ladies  "  gave  me  leave  to  satisfy 
myself  farther ;  and  desiring  her  to  hold  her  warm  hand  at  a 
convenient  distance  from  one  of  those  locks  taken  off  and 
placed  in  the  free  air,  as  soon  as  she  did  this,  the  lower  end  of 
the  lock,  which  was  free,  applied  itself  presently  to  her  hand."  ^ 

Again,  Newton  astonished  the  Eoyal  Society  by  the  descrip- 
tion of  an  experiment  with  a  round  piece  of  glass  set  in  a  brass 
ring  and  supported  by  it  about  one-eighth  of  an  inch  from  the 
table.  "Eubbing  a  pretty  while  the  glass  briskly  with  some 
rough  and  raking  stuff,  till  some  very  little  fragments  of  very 
thin  paper,  laid  on  the  table  under  the  glass,  began  to  be 
attracted  and  move  nimbly  to  and  fro  .  .  .  leaping  up  to  the 

It  is  worthy  of  remark  that  Halley  constructed  in  1686,  and  published  in 
the  Philosophical  Transactions,  1688,  No.  183,  the  earliest  wind  map.  It 
is  reprinted  in  Hellmann's  JSfeudrucke,  No.  8,  Berlin,  1897. 

1  E.  HoppE,  Entw.  d.  Lehre  v.  d.  Elektricitdt  his  auf  Haukshee,  Ham- 
burg, 1887,  p.  18. 

2  Boyle's  Works,  by  Peter  Shaw,  2d  ed.,  London,  1738,  Vol.  I.,  p.  506 
et  seq. ;  E.  Hoppe,  op.  cit.,  p.  17. 


96  A   HISTORY   OF   PHYSICS 

glass  and  resting  there  awhile ;  then  leaping  down  and  resting 
there ;  then  leaping  up  and  perhaps  down  and  up  again."  ^ 

Otto  von  Guericke  of  Magdeburg  generated  electricity  by 
holding  his  hands  against  a  rotating  sphere  of  sulphur.  This 
once  famous  contrivance  is  the  forerunner  of  the  friction 
electric  machine.  He  discovered  electric  induction  and  made 
a  number  of  other  interesting  observations,  but  his  specula- 
tions on  electricity  —  his  "mundane  virtues"  —  were  as  unfort- 
unate as  were  Gilbert's  cosmological  magnetic  theories. 

Boyle  made  an  important  experiment  showing  that  electric 
attraction  takes  place  through  a  vacuum.  In  1676  Picard, 
while  carrying  one  evening  a  mercury  barometer  from  the 
observatory  in  Paris  to  the  Porte  Saint  Michel,  saw  that  each 
motion  of  the  mercury  caused  a  glow  in  the  Torricellian 
vacuum.  The  cause  of  this  light  was  attributed  to  a  sub- 
stance called  mercurial  phosphorus.  This  name  was  suggested 
by  the  new  glow  phenomena  (phosphorescence)  of  phos- 
phorus, which  were  then  astonishing  the  scientific  world.  The 
origin  of  Picard's  light  was  studied  in  England  by  Francis 
Hauksbee.  He  let  air  rush  from  above  into  a  vacuum  through 
a  tube  dipping  into  a  basin  of  mercury  under  the  bell-jar, 
and  watched  the  air  blowing  the  mercury  up  "  with  violence 
against  the  sides  of  the  glass  that  held  it,  appearing  all  round 
as  a  body  of  fire,  made  up  of  abundance  of  glowing  globules 
descending  again  into  itself."^  Prom  this  and  other  tests 
with  mercury  Hauksbee  concluded  that  no  light  could  be 
obtained  without  motion  and  without  a  partial  vacuum.  He 
observed  that  attraction  accompanied  the  phenomena  and  con- 
cluded that  the  light  is  due  to  electricity.  He  was  the  first  to 
show  that  an  electric  charge  resides  only  on  the  surface  of  a 
body,  and  that  metals  may  become  electrified  by  friction. 

1  T.  Birch,  Hist,  of  Boyal  Society,  Vol.  III.,  London,  1757,  p.  250  ; 
E.  HoppE,  op.  cit..  p.  14. 

2  Phil.  Trans.,  1705,  No.  303,  p.  2129 ;  Hoppe,  op.  cit.,  p.  21. 


SOUND  97 

SOUND 

Vibratory  strings  were  made  the  subject  of  investigation  by 
Galileo  and  Marin  Mersenne.  Galileo  pointed  out  the  depend- 
ence of  pitch  upon  the  number  of  vibrations  perceived  in  unit 
of  time.  Mersenne  noticed  that  a  string  gives,  besides  its 
fundamental,  two  overtones.  At  Oxford,  William  Nohle  and 
Thomas  Pigott  showed  by  paper  riders  put  at  different  places 
on  a  vibrating  string  that  it  vibrates  not  only  as  a  whole,  but 
also  in  halves,  thirds,  etc.^  Mersenne  determined  the  velocity 
of  sound  in  air  by  the  difference  in  time  between  the  flash 
and  the  report  of  fire-arms  at  known  distances.  He  got  1380 
feet  per  second.  Pierre  Gassendi  (1592-1655)  used  cannon  as 
well  as  pistols,  and  disproved  the  peripatetic  tenet  that  the 
velocity  depends  upon  its  source  and  its  pitch.  His  test  gave 
1473  Paris  feet.  The  illustrious  members  of  the  Paris  Acad- 
emy, D.  Cassini,  Picard,  Eomer,  Huygens,  found  the  value 
1172  Paris  feet  per  second. 

ISTewton  published  in  his  Prindpia  (Book  II.,  Props.  XLYIII., 
XLIX.,  L.)  a  theoretical  deduction  for  the  velocity  of  sound. 
He  concluded  that  this  velocity  varied  directly  as  the  square 
root  of  the  "  elastic  force,"  and  inversely  as  the  square  root  of 
the  "  density  of  the  medium  " ;  the  velocity  is  "  equal  to  that 
which  heavy  bodies  acquire  by  falling  with  an  equally  accel- 
erated motion,  and  in  their  fall  describing  half  the  altitude 
^,"  where  A  is  the  height  of  a  homogeneous  atmosphere, 
taken  as  29,725  feet.  This  gave  979  feet  as  the  velocity, 
while  experiment  indicated  about  1142  English  feet.  Newton 
threw  out  conjectures  as  to  the  cause  of  the  discrepancy 
between  the  experimental  and  theoretical  values,  but  the  true 
explanation  was  given  over  a  century  later  by  Pierre  Simon 
Laplace  (1749-1827).     Newton  failed  to  take  into  account  the 

1  Phil.  Trans. ,  1677  ;  Heller,  Vol.  II.,  p.  339. 


98  A    HISTORY    OF   PHYSICS 

changes  of  elasticity  due  to  the  heat  of  compression  and  the 
cold  of   rarefaction.      His   expression    amounts   to   v=yj-^-y 

-1^,  where  p  is  the 
pressure  of  the  air  and  d  its  density. 


THE  EIGHTEENTH   CENTURY 

The  progress  of  physics  during  the  first  eighty  years  of  the 
seventeenth  century  was  truly  extraordinary.  Nothing  like  it 
is  seen  during  the  earlier  epochs  of  human  history;  nothing 
like  it  is  exhibited  during  the  eighteenth  century.  The  names 
of  Galileo,  Guericke,  Boyle,  and  Newton  adorn  the  period 
when  experiment  assumed  a  place  of  supreme  authority.  In 
the  eighteenth  century  there  comes  a  reaction.  On  the  whole, 
speculation  is  less  effectively  restrained  and  guided  by  ex- 
periment. 

Another  important  cause  makes  the  period  less  brilliant. 
It  brought  forth  few  great  experimental  physicists  —  none 
of  such  transcendent  genius  as  Galileo,  Huygens,  Newton. 
Mathematics  and  mathematical  astronomy  were  enriched  dur- 
ing the  eighteenth  century  by  the  remarkable  researches  of  the 
Bernoullis,  Euler,  Clairaut,  D'Alembert,  Lagrange,  Laplace, 
but  physics  proper  was  cultivated  by  men  of  more  limited 
powers. 

MECHANICS 

The  mechanical  principles,  as  stated  by  Newton,  suffice  to 
explain  any  practical  problem,  whether  in  statics  or  dynamics. 
Nevertheless,  it  has  been  found  convenient  to  deduce  partic- 
ular laws  by  which  certain  groups  of  problems  may  be  treated 
by  routine.  As  examples,  we  cite  "  D'Alembert's  Principle  " 
and  the  laws  of  the  "Conservation  of  Momentum,"  "Conserva- 
tion of  the   Centre   of   Gravity,"   "  Conservation   of   Areas." 

99 


100  A  HISTORY   OF  PHYSICS 

The  eighteenth  century  has  contributed  much  toward  the 
development  of  these  principles,  enabling  mechanical  phe- 
nomena to  be  viewed  from  new  standpoints.  But  their  sub- 
jects, together  with  the  analytical  development  of  mechanics, 
lie  outside  the  scope  of  this  work.^ 

It  only  remains  for  us  to  mention  the  new  contrivance, 
invented  by  George  Atwood  (1746-1807),  for  the  study  of  the 
laws  of  falling  bodies.  Galileo  had  retarded  the  velocity  of 
falling  bodies  by  means  of  the  inclined  gutter,  and  thereby 
facilitated  experimentation.  Atwood  accomplished  this  end 
by  suspending  two  weights  by  a  thread  over  an  easily  running 
pulley. 

Atwood  was  a  fellow  and  tutor  of  Trinity  College,  Cambridge, 
where  his  public  lectures  in  experimental  philosophy  were 
remarkable  both  for  the  fluent  ease  of  delivery  and  for  the 
ingenuity  of  their  experimental  illustrations.  His  influence 
on  the  scientific  studies  of  the  university  was  great.  As  a 
writer  he  was  less  gifted  than  as  a  lecturer.^  In  1784  he 
published  a  treatise  On  the  Rectilinear  Motion  and  Rotation  of 
Bodies.  The  description  of  Atwood's  machine,  *iven  therein, 
is  not  the  earliest  account  of  it  made  public.  Atwood  invented 
the  machine  several  years  previously,  and  a  full  account  of  it 
was  given  by  J.  H.  de  Magellan,  a  "  gentil-homme  Portugais  '* 
residing  in  England,  in  a  letter  addressed  in  French  to  A. 
Volta,  professor  at  the  University  of  Pavia,  who  had  ordered  a 
machine  to  be  sent  to  him  from  England.  The  letter  was 
printed  in  London  in  1780. 

1  The  reader  may  consult  E.  Mach,  The  Science  of  Mechanics.,  trans, 
by  T.  J.  McCoRMACK,  Chicago,  1893;  E.  Ddhring,  Krit.  Gesch.  d, 
allgem.  Princ.  d.  3Iechanik,  Leipzig,  1887. 

2  Die.  Nat.  Biog.  See  also  T.  Young's  Misc.  Works^  Vol.  II.,  pp. 
617-623. 


LIGHT  101 

LIGHT 

During  the  seventeenth  century  we  have  witnessed  a  conflict 
between  two  theories  of  light.  We  have  seen  how  Newton, 
from  the  facts  then  known,  balanced  the  arguments  for  and 
against  each  theory  and  hesitatingly  decided  in  favour  of  the 
emission  theory,  while  on  the  continent  his  great  contempo- 
rary, Huygens,  advocated  the  wave  theory.  Says  S.  P.  Lang- 
ley,  "These  two  great  men,  then,  each  looked  around  in  the 
darkness  as  far  as  his  light  carried  him.  All  beyond  that  was 
chance  to  each;  and  fate  willed  that  Newton,  whose  light 
shone  farther  than  his  rival's,  found  it  extend  just  far  enough 
to  show  the  entrance  to  the  wrong  way.  He  reaches  the  con- 
clusion that  we  all  know;  one  not  only  wrong  in  regard  to 
light,  but  which  bears  pernicious  results  on  the  whole  theory 
of  heat,  since  light,  being  conceded  to  be  material,  radiant  heat, 
if  affiliated  to  light,  must  be  regarded  as  material  too ;  and 
Newton's  influence  is  so  permanent,  that  we  shall  see  this 
strange  conclusion  drawn  by  the  contemporaries  of  Herschel 
from  his  experiments  made  a  hundred  years  later.  It  would 
seem  then  that  the  result  of  this  unhappy  corpuscular  theory 
was  more  far-reaching  than  we  commonly  suppose."  ^ 

The  history  of  physics  affords  two  capital  instances  of  the 
danger  to  science  of  placing  excessive  confidence  in  authority, 
no  matter  how  great.  The  unscientific  physical  speculations 
of  Aristotle  held  the  world  bound  within  their  grasp  for  two 
thousand  years ;  the  unfortunate  corpuscular  theory  of  New- 
ton controlled  scientific  thought  for  over  a  century.  The  phi- 
losophy of  the  eighteenth  century  was  actually  farther  from 
the  truth  than  that  of  Newton's  predecessors.  Newton's 
repute  seemed  to  act  like  a  spell.     "  The  adoption  of  phlo- 

1  S.  P.  Langley,  The  History  of  a  Doctrine^  1888,  p.  4,  delivered  at 
the  Cleveland  meeting  of  the  A.  A.  A.  S. 


102  A    HISTORY    OF   PHYSICS 

giston  was,  as  we  may  reasonably  infer,  facilitated  by  it,  and 
remotely  Newton  is  perhaps  also  responsible  in  part  for  the 
doctrine  of  caloric  a  hundred  years  later.  After  him,  at  any 
rate,  there  is  a  great  backward  movement."  ^ 

The  only  prominent  writer  of  the  eighteenth  century  who 
advocated  the  undulatory  theory  was  Leonliard  Euler  (1707- 
1783).^  He  advanced  only  theoretical  considerations  in  its 
favour,  and  convinced  no  one.  In  1750  he  published  his  Lettres 
d,  une  Princesse  d^AUemagne  sur  quelques  sujets  de  physique. 
The  German  translator  of  this  work  thought  it  necessary  to 
interpolate  explanations,  lest  some  innocent  reader  might  be 
led  to  believe  in  a  theory  which  was  not  held  now  (1792)  "  by 
a  single  physicist  of  prominence."^  Euler  explained  diversity 
in  colours  by  the  difference  in  duration  of  vibrations.  He  made 
the  conjecture  that  the  different  media  of  the  eye  had  the 
property  of  preventing  the  dispersion  of  colours,  and  suggested 
that  lenses  be  prepared  out  of  two  different  substances,  with  the 
view  of  removing  chromatic  defects.  He  had  a  theory  as  to 
how  this  might  be  done,  but  was  not  able  actually  to  produce 
a  lens  free  from  colour.  This  failure  he  attributed  to  the  diffi- 
culty of  accurate  construction.  The  only  good  which  resulted 
from  his  efforts  was  this :  he  excited  the  curiosity  of  Samuel 
Klingenstierna,  professor  at  Upsala,  who  began  to  repeat  New- 
ton's experiments  on  achromatism  and  arrived  at  results  at 
variance  with  Newton's.  At  this  stage,  John  Dollond,  a  Lon- 
don optician,  began  a  series  of  tests.  They,  too,  went  contrary 
to  Newton's.  Dollond  then  tested  different  kinds  of  glass,  and 
in  1757  wrote  a  letter  to  Klingenstierna,  in  which  he  points 
out  that  the  ratio  of  the  sine  of  incidence  to  the  sine  of  the 

1  S.  P.  Langlet,  The  History  of  a  Doctrine^  1888,  p.  5. 

2  Papers  thereon  appear  in  Memoiren  der  Berliner  Akademie,  1746, 
1752. 

8  RosENBERGER,  7.  JSfewton  u.  s.  FhysiJc.  Princ,  1895,  p.  332. 


LIGHT  101 

mean  angle  of  refraction  is  1.53  for  crown  glass  and  1.583  for 
flint  glass.^  Hence  lie  concluded  that  achromatism  must  be 
possible  in  lenses.  The  practical  realization  of  this  idea 
proved  to  be  difficult  and  required  (to  use  his  own  words)  "a 
resolute  perseverance."  ^  In  1758  he  at  last  succeeded,  and 
presented  an  achromatic  telescope  to  the  Royal  Society.  It 
created  a  sensation  throughout  Europe.  -  Dollond's  success 
seemed  to  disprove  Euler's  theory  of  dispersion  and  caused 
him  much  embarrassment.  "If  my  theory  is  right/'  Euler 
says,  "then  it  follows  that  Mr.  Dollond's  object-lenses  are  not 
free  from  dispersion  of  colours,  to  which,  however,  Mr.  Short 
explicitly  testifies.  It  is  as  difficult  for  me  to  call  in  question 
so  solemn  a  testimony,  as  it  is  to  give  up  a  theory  which 
seems  to  be  so  well  founded."  ^  Eor  some  time  he  persisted  in 
the  belief  that  experiments  with  Dollond's  objectives,  made  in 
the  dark,  would  show  colour  effects. 

After  Dollond's  death,  in  1761,  his  son,  Peter  Dollond,  man- 
ufactured (in  partnership  with  the  mechanic  Ramsden)  re- 
fractors of  great  merit.  After  repeated  failures,  achromatic 
lenses  came  to  be  applied  successfully  also  to  microscopes. 

The  achromatic  telescope  greatly  facilitated  the  growth  of 
modern  astronomy.  How  great  an  advantage  was  secured  be- 
comes the  more  apparent  when  we  remember  that  Huy gens' s 
method  of  removing  colour  effects  by  the  use  of  lenses  of  great 
focal  length  led  to  the  construction  by  him  of  very  long  tube- 
less  refractors  (the  objectives  being  mounted  on  high  poles), 
which  were  exceedingly  clumsy  and  at  the  same  time  yielded 
inferior  optical  results.  One  object  lens  presented  by  him  to 
the  Royal  Society  had  a  focal  length  of  123  feet. 

When  Dollond's  telescopes  had  become  famous,  the  claims 

1  H.  Servus,  Geschichte  des  Fernrohrs^  1886,  p.  83. 

2  Consult  Phil.  Trans.,  Vol.  50,  1758,  p.  733. 

*  Mem-  d.  Berliner  Akademie,  1762,  p.  260  ;  Servus,  op.  cit.,  p.  85. 


104  A   HISTORY   OF   PHYSICS 

of  another  man  were  laid  before  the  public.  As  early  as  1729 
Cliester  More  Hall,  of  More  Hall  in  Essex,  while  studying  the 
mechanism  of  the  human  eye^  was  led  to  the  design  of  lenses 
without  colour.  He  employed  several  working  opticians  to 
grind  his  lenses,  and  several  object-glasses  were  completed. 
But  he  never  published  any  account  of  his  labours.  Perhaps 
he  kept  them  secret,  hoping  to  perfect  his  instruments  still 
further.  At  any  rate  the  invention  was  lost,  and  Dollond's 
work  was  independent  of  Hall's.^ 

Contemporaneous  with  the  early  development  of  the  achro- 
matic telescope  is  the  construction  of  large  reflecting  tele- 
scopes. Again  England  displayed  superior  skill.  In  1723, 
about  half  a  century  after  Newton  made  his  reflectors,  John 
Hadley  presented  to  the  Eoyal  Society  an  instrument  six  feet 
long.  It  equalled  in  performance  the  Huygenian  refractor 
123  feet  in  length!  Eurther  progress  in  the  design  of  con- 
cave mirrors  was  made  by  James  Short  of  Edinburgh,  and 
especially  by  William  Herschel  (1738-1822).  To  improve  the 
*^  space-penetrating  power  '^  Herschel  increased  the  light-gath- 
ering power  by  the  use  of  larger  mirrors.  He  experimented 
in  the  shaping  and  polishing  of  concave  mirrors  with  an  en- 
thusiasm and  skill  never  surpassed.  Mirrors  of  10,  20,  30 
feet,  and  finally  one  of  40  feet  focal  length,  left  his  hands. 
The  last,  completed  in  1789,  was  four  feet  in  diameter  and 
weighed  2500  pounds.  The  telescope  led  to  Herschel's  dis- 
covery of  the  two  Saturnian  satellites  nearest  to  the  ring.  Only 
two  reflectors  larger  than  this  have  ever  been  constructed.  In 
1745  was  completed  by  Lord  Rosse  at  Parsonstown  in  Ireland 
a  gigantic  reflector,  with  a  mirror  six  feet  across  and  a  tube 

1  D.  Brewster,  Life  of  Sir  I.  Newton,  New  York,  1831,  pp.  64-67. 
For  further  details  on  Hall  and  achromatism,  consult  the  article  ' '  Tele- 
scope" in  the  Encyclopaedia  Britannica^  9th  ed.,  and  the  article 
"Optics"  in  the  Edinburgh  Encyclopcedia,  p.  607,  not©. 


HEAT  105 

58  feet  long  and  seven  feet  in  diameter.  So  large  was  this 
tube  that  Dean  Peacock  walked  through  it  once  with  uplifted 
umbrella.^  This  "light-grasper"  displayed  celestial  objects 
with  extraordinary  splendour.  "JSTever  in  my  life,"  exclaims 
Sir  James  South,  "  did  I  see  such  glorious  siderial  pictures  ! " 
The  second  large  reflector,  with  a  mirror  over  61  inches  in 
diameter,  is  now  being  finished  for  the  University  of  America 
in  Washington. 

There  are  two  objections  to  reflecting  telescopes  which  have 
not  yet  been  successfully  overcome.  The  great  weight  of  a 
large  speculum  mirror  makes  it  liable  to  distortion.  The 
delicate  lustre  of  its  surface  cannot  be  preserved  permanently, 
and  must  be  restored  by  the  difficult  operation  of  repolishing.^ 

HEAT 

Guillaume  Amontons  (1663-1705)  effected  in  1702  an  improve- 
ment of  Galileo's  air  thermometer.  In  his  youth  Amontons 
became  deaf ;  but  this  was  not  regarded  by  him  as  an  affliction, 
since  it  permitted  scientific  pursuits  with  less  molestation  from 
the  outer  world.  He  held  a' government  position  in  Paris.  His 
air  thermometer  was  of  constant  volume  and  consisted  of  a 
U-shaped  tube  with  the  shorter  arm  ending  in  a  bulb  and 
the  longer  measuring  45  inches.     Degrees  of  temperature  were 

1  A.  M.  Clerke,  a  Popular  History  of  Astronomy,  New  York,  1893, 
pp.  145,  147. 

2  Consult  further  the  history  of  the  telescope  by  C.  S.  Hastings  in 
Smithsonian  Beport,  1892  ;  also  George  E.  Hale,  "  On  the  Comparative 
Value  of  Refracting  and  Reflecting  Telescopes  for  Astrophysical  Investi- 
gations," Astrophysical  Journal,  Vol.  V.,  1897,  pp.  119-131.  Students 
interested  in  the  history  of  anamorphosis  during  the  sixteenth  and  seven- 
teenth centuries  may  consult  H.  Ruoss,  "  Geschichte  der  optischen  und 
katoptrischen  anamorphosen,"  Zeitsch.  d.  Math.  u.  Fhys.,  Vol.  39,  1894, 
Hist.  Lit.  Abth.,  p.  1. 


106  A   HISTORY   OF   PHYSICS 

indicated  by  tlie  height  (in  inches)  of  the  mercury  column  in 
the  longer  arm  necessary  to  keep  the  volume  constant.  The 
instrument  was  intended  as  a  standard,  by  which  a  mercury 
thermometer  in  Paris,  say,  could  be  compared  with  one  in  St. 
Petersburg  without  the  necessity  of  transmitting  thermometers 
from  one  place  to  the  other.  But  the  invention  met  with  little 
favour.  He  chose  the  boiling-point  of  water  as  a  fixed  point, 
but,  being  unaware  of  the  dependence  of  the  boiling-point 
upon  air  pressure,  he  could  not  attain  extreme  accuracy.-^  It 
is  an  interesting  fact  that  Amontons's  researches  amount  to  an 
experimental  proof  of  the  law  of  gases  now  named  after 
Charles  and  Gay-Lussac,  and  that  he  first  arrived  at  the 
notion  of  absolute  temperature.  "  It  appears,"  says  he,  "  that 
the  extreme  cold  of  this  thermometer  is  that  which  would 
reduce  the  air  by  its  spring  to  sustain  no  load  at  all,  which 
would  be  a  degree  of  cold  much  more  considerable  than  what 
is  esteemed  very  cold."  From  Amontons's  data  the  absolute 
zero  in  our  centigrade  scale  is  found  to  be  —  239.5°.  Lambert, 
who  repeated  Amontons's  experiments  with  greater  accuracy,^ 
obtained  data  yielding  —  270.3°.  The  value  now  accepted  is 
—  272.8°.  Lambert  uses  this  language :  "  Now  a  degree  of 
heat  equal  to  zero  is  really  what  may  be  called  absolute  cold. 
Hence  at  absolute  cold  the  volume  of  the  air  is  zero,  or  as 
good  as  zero.  That  is  to  say,  at  absolute  cold  the  air  falls 
together  so  compactly  that  its  parts  absolutely  touch,  that  it 
becomes,  so  to  speak,  water-proof." 

Stimulated  by  Amontons's  researches  Gabriel  Daniel  Fahren- 

1  E.  Gerland,  "Ueber  Amontons'  und  Lambert's  Verdienste  um  die 
Thermometrie,"  Zeitsch.  f.  Instrumentenkunde^  Vol.  VIII.,  1888,  pp. 
319-322.  Abstract  given  in  Poske's  Zeitschrift,  Vol.  II.,  1889,  pp.  142, 
143. 

2  Lambert,  Pyrometrie,  Berlin,  1779,  p.  29 ;  E.  Geriand,  Instrument 
tenkunde,  Vol.  VIII.,  p.  322. 


HEAT  107 

Jieit  (1686-1736)  began  to  study  the  accurate  construction  of 
thermometers.  He  was  a  native  of  Danzig,  but  went  to 
Amsterdam  to  secure  a  business  education;  he  became  inter- 
ested in  physics,  and  travelled  in  England,  Denmark,  and 
Sweden.  He  was  a  manufacturer  of  meteorological  instru- 
ments. That  he  attained  considerable  celebrity  is  evident 
from  his  election  to  the  Eoyal  Society  of  London  in  1724. 
The  same  year  he  contributed  to  the  Philosophical  Transac- 
tions five  short  papers  in  Latin.  Therein  he  revealed  for  the 
first  time  his  process  of  making  thermometers.^  Fahrenheit 
was  in  communication  with  Olaf  Komer,  whom  he  probably 
visited  in  Copenhagen.  During  the  cold  winter  of  1709  both 
are  said  to  have  taken  records  of  temperatures. 

Fahrenheit  was  greatly  interested  in  Amontons's  observa- 
tions of  the  constancy  of  the  boiling-point  (previously  observed 
by  Huygens,  Newton,  and  Halley).  Curious  to  know  how 
other  liquids  would  behave,  he  made  a  series  of  tests,  and 
found  that,  like  water,  each  had  a  fixed  boiling-point.^  Later 
he  noticed  that  the  boiling-point  varied  with  a  change  in 
atmospheric  pressure.^  Attention  to  this  fact  contributed 
vastly  towards  exact  thermometry.  Fahrenheit  deserves  great 
credit  for  first  bringing  about  the  general  use  of  mercury 
in  thermometers.  (The  earliest  mercury-in-glass  thermom- 
eter, it  will  be  remembered,  is  due  to  Ismael  Boulliau, 
1659.)  The  success  of  Fahrenheit's  mercury  thermometers 
was  largely  due  to  a  method  which  he  invented  for  cleaning 
the  mercury. 

Fahrenheit  made  two  kinds  of  thermometers,  —  the  one  filled 

1  The  five  papers,  together  with  articles  on  thermometry  by  Reaumur 
and  Celsius,  are  brought  out  in  German  translation  in  Ostwald^s  Klassi- 
ker,  No.  57,  Leipzig,  1894. 

2  Phil.  Trans.,  Vol.  30,  1724,  pp.  1-3 ;  Ostwald's  Klass.,  No.  57,  p.  3. 

3  PML  Trans,,  Vol.  33,  pp.  179,  180  j  OstwaWs  Klass.,  No.  67,  p.  17. 


108  A   HISTORY   OF   PHYSICS 

with  spirit  of  wine,  the  other  with  mercury.  Various  lengths 
were  chosen  for  the  stem.  In  1724  he  wrote  as  follows  :  "  The 
scale  of  those  thermometers  which  are  used  only  in  meteoro- 
logical observations  begins  with  0  and  ends  with  96.  This 
scale  depends  upon  the  determination  of  three  fixed  points, 
which  are  obtained  as  follows :  the  first,  the  lowest,  ...  is 
found  by  a  mixture  of  ice,  water,  and  sal-ammoniac  or  sea 
salt;  if  the  thermometer  is  dipped  in  this  mixture,  then  the 
liquid  falls  to  the  point  marked  0.  This  experiment  succeeds 
better  in  winter  than  in  summer.  The  second  point  is  ob- 
tained, if  water  and  ice  are  mixed  without  the  salts  just 
mentioned;  if  the  thermometer  is  dipped  in  this  mixture,  it 
will  stand  at  32°  .  .  .  ;  the  third  point  is  at  the  96th  degree,  and 
the  alcohol  expands  to  that  point  if  the  thermometer  be  held 
in  the  mouth  or  armpit  of  a  healthy  person."  ^ 

His  earliest  thermometers  were  constructed  differently. 
Taking  only  two  fixed  points,  the  first  and  last  points 
mentioned  above,  he  divided  the  interval  in  180  equal  parts, 
but  he  placed  0  half-way  between,  so  that  there  were  90°  to 
the  upper  and  90°  to  the  lower  fixed  point.  After  about  1714 
he  divided  the  interval  into  24  equal  parts.  According  to  his 
contemporary,  Boerhave,  this  change  was  made  on  Eomer's 
suggestion.  The  0  was  placed  this  time  at  the  lower  fixed 
point.  The  degrees,  being  found  too  large,  were  subdivided 
into  four  parts.  Thereby  the  fixed  points  came  to  be  desig- 
nated 0  and  96,  respectively.  It  is  not  unlikely  that  he  used 
the  melting  temperature  of  ice  as  a  "  check,"  to  see  whether 
his  alcohol  thermometers  gave  consistent  readings  for  inter- 
vening points.  This  would  explain  how  it  happened  that, 
unlike  every  one  else,  he  had  three  fixed  points.  Later,  when 
he  began  to  use  mercury,  he  took,  in  place  of  the  temperature 

1  Ostwald''s  Klassiker,  No.  57,  pp.  6,  7. 


HEAT  109 

of  the  human  body,  the  boiling-point  of  water.     On  his  scale 
this  happened  to  he  at  212  °.^ 

While  the  Fahrenheit  thermometers  were  adopted  by  the 
Dutch  and  English,  other  nations  were  slow  to  appreciate 
their  value.  In  France  Eeaumur  designed  thermometers. 
Rene  Antoine  Ferchault,  Seigneur  Reaiimui^,  des  Angles  et  de  la 
Bermodih-e  (1683-1757),  was  born  at  Kochelle  and  died  at 
Bermodiere.  He  is  known  for  his  researches  in  zoology, 
botany,  and  physics.  He  was  not  familiar  with  Fahrenheit's 
achievements.  Dissatisfied  with  Amontons's  air  thermometer 
^the  only  thermometer  which  he  considered  at  all  fit  for  use) 
and  strongly  opposed  to  the  use  of  mercury  on  account  of 
its  small  coefficient  of  expansion,  he  endeavoured  to  construct 
instruments  with  spirit  of  wine  which  should  be  convenient 
and  yet  reach  the  desired  degree  of  accuracy.  His  experi- 
ments accidentally  led  to  the  beautiful  observation  of  the  con- 
traction in  volume  which  may  result  on  the  mixing  of  liquids.^ 
He  found  that  spirit  of  wine  (mixed  with  \  water)  expanded 
between  the  freezing  and  the  boiling  temperatures  of  water 
from  1000  to  1080  volumes;  so  he  divided  the  intervening 
distance  on  the  stem  into  80  parts.  But  Eeaumur's  ther- 
mometers did  not  turn  out  well.  All  sorts  of  incredible 
readings  were  reported,  and  different  instruments  did  not 
agree.  Jean  Antoine  Nollet  endeavoured  to  improve  Reaumur's 
thermometers,  but  more  was  achieved  by  Jean  Andre  Deluc 
(1727-1817)  of  Geneva.  He  returned  to  the  use  of  mercury, 
and  emphasized  its  advantages  by  arguments  so  powerful,  that 


1  E.  Gerland,  Das  Thermometer^  Berlin,  1885,  pp.  14,  15.  Eor  further 
details  on  Fahrenheit's  career,  see  Albert  Momber  in  Altpreussische 
Monatsschrift,  Vol.  24  (Provinzialbldtter,  Vol.  89),  Konigsberg,  1887, 
pp.  138-156. 

2  OstwaWs  Klassiker,  No.  57,  pp.  100-116,  127.  Reaumur's  three 
articles  on  thermometers  appear  in  German  translation  on  pages  19-116. 


110  A    HISTORY   OF   PHYSICS 

a  physicist  enthusiastically  exclaimed,  "  Surely  nature  has 
given  us  this  mineral  for  the  making  of  thermometers."  ^ 

On  the  other  hand  MicJieli  du  Crest,  another  scientist  of 
Geneva,  had  no  use  for  mercury,  except  to  calibrate  capillary 
tubes.  He  and  De  VIsle  in  St.  Petersburg  introduced  this 
process  about  the  same  time.^  In  1757  Du  Crest  raised  the 
boiling-point  of  alcohol  by  subjecting  it  to  the  pressure  of  air 
enclosed  in  the  upper  enlarged  end.  He  anticipated  Celsius 
in  the  design  of  a  centesimal  scale.  Eejecting  the  temperature 
of  freezing  water  as  a  fixed  point,  he  chose  the  temperature  of 
the  earth  as  determined  in  the  cellar  at  the  Paris  Observatory, 
84  feet  deep.  This  was  not  a  new  idea  with  him.  Boyle 
referred  to  the  constancy  of  temperature  in  deep  cellars.  Por 
thermometric  purposes  this  temperature  was  first  used  by 
Dalence.  Du  Crest  divided  the  interval  between  this  and 
the  boiling-point  into  100  steps,  and  thereby  obtained  degrees 
agreeing  closely  with  Eeaumur's.  Part  of  his  physical  re- 
searches was  carried  on  during  his  twenty  years  of  political 
imprisonment. 

Centigrade  scales  were  adopted  after  Du  Crest  by  Celsius 
and  Stromer.  Andreas  Celsius  (1701-1744)  was  professor  of 
astronomy  at  Upsala.  His  researches  are  mainly  astronomical. 
A  publication  of  1742^  contains  the  description  of  his  ther- 
mometer, with  100  divisions  between  the  freezing-  and  the 
boiling-point  of  water.  The  latter  point  was  marked  0°,  the 
former  100°.  The  inversion  of  the  scale,  making  the  freezing- 
point  0°  and  the  boiling-point  100°,  was  effected  eight  years 


1  Deluc,  Becherches  sur  les  Modifications  de  V Atmosphere^   Geneve^ 
1772,  p.  330.     E.  Gerland,  op.  cit.,  p.  20. 

2  J.  H.  Graf,  Das  Lehen  und  Wirken  des  Physikers  und  Geoddten 
Jacques  Barthelemy  Micheli  du  Crest.,  Bern,  1890,  p.  114. 

3  Ahhandl.  d.  schwedisch.  Akademie,  Vol.  IV.,  pp.  197-205 ;  trans,  in 
OstwaWs  Elass.,  No.  57,  pp.  117-124. 


HEAT  111 

later  by  Marten  Stromer,  a  colleague  of  Celsius.  The  final 
form  of  our  modern  centigrade  scale  is,  therefore,  not  that  of 
Celsius,  but  that  of  Stromer.^ 

The  number  of  different  thermometric  scales  in  actual  use 
in  the  eighteenth  century  increased  greatly.  George  Martine 
in  1740  mentions  13  of  them ;  J.  H.  Lambert  in  1779  enumer- 
ates 19.^  All  but  three  of  them  have  passed  into  oblivion. 
Would  that  the  centigrade  scale  were  the  sole  survivor !  In 
England  and  America  the  Fahrenheit  scale  predominates ; 
in  Germany  the  Eeaumur;  in  France  the  Celsius.  Among 
scientific  men  the  last  has  found  almost  universal  acceptance. 

The  earliest  thermometer  depending  on  the  expansion  and 
contraction  of  metallic  rods  was  invented  about  1747  by  Pieter 
van  MusscJienbroek  of  Leyden.  It  was  improved  by  Jean  Theo- 
phile  Desaguliers.  About  thirty-five  years  later  came  the 
pyrometer  of  Josiah  Wedgwood,^  by  which  the  high  tempera- 
tures of  a  furnace  were  measured  by  the  diminution  in  bulk 
of  a  block  prepared  from  a  pure  fire-clay  according  to  certain 
directions.* 

In  1705  there  was  invented  the  first  important  device  for 
the  practical  application  of  steam  power.  For  over  1000  years 
after  Heron's  eolipile  no  progress  had  been  made.    During  the 

1  Unless  indeed  the  credit  of  inversion  be  given  also  to  Christin,  a  pro- 
fessor in  Lyons.  See  Poggetidorfs  Annalen^  Vol.  157,  1876,  p.  352. 
Celsius  and  Stromer  may  liave  been  prompted  to  make  their  improvements 
in  thermometry  by  the  botanist  Linne^  who  once  wrote  in  a  letter,  "  I  was 
the  first  who  planned  to  make  our  thermometers  in  which  0  is  the  freez- 
ing-point and  100  the  degree  of  boiling  water."  Comptes  Hendics, 
Vol.  18,  p.  1063.  It  will  be  remembered  that  the  earliest  suggestion  of 
the  use  of  these  temperatures  as  fixed  points  was  made  by  Huygens. 

2  Martine,  Essays  medical  a^id  philosophical,  London,  1740;  Lambert, 
Pyrometrie,  Berlin,  1779. 

3  Phil.  Trans.,  Vol.  72,  1782;  Vol.  74,  1784. 

*  G.  T.  Halloavay,  "The  Evolution  of  the  Thermometer,"  Science 
Progress,  Vol.  IV.,  1895-1896,  p.  417. 


112  A   HISTOKY   OF   PHYSICS 

seventeenth  century  steam-fountains  were  designed,  but  they 
were  merely  modifications  of  Heron's  engine,  and  were  probably 
applied  only  for  ornamental  purposes.-^  Some  effort  was  also 
made  by  Morland,  Papin,  and  Savery  to  construct  practical 
machines  for  the  raising  of  water  or  driving  of  mill-works. 
The  first  successful  attempt  to  combine  the  principles  and 
forms  of  mechanism  then  known  into  an  economical  and  con- 
venient machine  was  made  by  TJiomas  Newcomen,  a  blacksmith 
of  Dartmouth,  England.  It  is  probable  that  he  knew  of 
Savery's  engine ;  Savery  lived  only  fifteen  miles  from  the 
residence  of  liewcomen.  Assisted  by  John  Galley,  Newcomen 
constructed  an  engine  —  an  "atmospheric  steam-engine."  A 
patent  was  secured  in  1705.  In  1711  such  a  machine  was  set 
up  at  Wolverhampton  for  the  raising  of  water.  Steam  passing 
from  the  boiler  into  the  cylinder  held  the  piston  Up  against 
the  external  atmospheric  pressure  until  the  passage  between 
the  cylinder  and  boiler  was  closed  by  a  cock.  Then  the  steam 
in  the  cylinder  was  condensed  by  a  jet  of  water.  A  partial 
vacuum  was  formed  and  the  air  above  pressed  the  piston  down. 
This  piston  was  suspended  from  one  end  of  an  overhead  beam, 
the  other  end  of  the  beam  carrying  the  pump-rod.  Desaguliers 
tells  the  story  that  a  boy,  Humphrey  Potter,  who  was  charged 
with  the  duty  of  opening  and  closing  the  stop-cock  between 
the  boiler  and  cylinder  for  every  stroke,  contrived  by  catches 
and  strings  an  automatic  motion  of  the  cock.^  The  fly-wheel 
was  introduced  in  1736  by  Jonathan  Hulls.  The  next  great 
improvements  were  introduced  in  Scotland  by  James  Watt 
(1736-1819).  He  was  educated  as  a  mathematical  instrument 
maker.  In  1760  he  opened  a  shop  in  Glasgow.  Becoming 
interested  in  the  steam-engine  and  its  history,  he   began   to 

1  R.  H.  Thukston,  a  History  of  the  Growth  of  the  Steam-engine,,  New 
York,  1893,  p.  20. 

2  Thurston,  op.  ciY.,  p.  61. 


HEAT  113 

experiment  in  a  scientific  manner.  He  took  up  chemistry  and 
was  assisted  in  his  studies  by  Dr.  Black,  the  discoverer  of 
"  latent  heat."  ^  Observing  the  great  loss  of  heat  in  the  ISTew- 
comen  engine  due  to  the  cooling  of  the  cylinder  by  the  jet 
of  water  at  every  stroke,  he  began  to  think  of  means  to  keep 
the  cylinder  "  always  as  hot  as  the  steam  that  entered  it."  He 
has  told  us  how,  finally,  the  happy  thought  securing  this  end 
occurred  to  him  :  "  I  had  gone  to  take  a  walk  on  a  fine  Sabbath 
afternoon.  I  had  entered  the  Green  by  the  gate  at  the  foot 
of  Charlotte  Street,  and  had  passed  the  old  washing-house.  I 
was  thinking  upon  the  engine  at  the  time,  and  had  gone  as  far 
as  the  herd's  house,  when  the  id-ea  came  into  my  mind  that, 
as  steam  was  an  elastic  body,  it  would  rush  into  a  vacuum, 
and,  if  a  communication  were  made  between  the  cylinder  and 
an  exhausted  vessel,  it  would  rush  into  it,  and  might  be  there 
condensed  without  cooling  the  cylinder."^  The  piston  was 
now  moved  by  the  expansion  of  steam,  not  by  air  pressure, 
as  in  Newcomen's  engine.  Watt  introduced  a  separate  con- 
denser, a  steam-jacket,  and  other  improvements.  He  de- 
servedly commands  a  preeminent  place  among  those  who  took 
part  in  the  development  of  the  steam-engine. 

During  the  previous  century  the  leading  scientists  saw  more 
or  less  clearly  that  heat  was  due  to  molecular  motion.  But 
this  correct  view  was  finally  abandoned  in  the  eighteenth 
century  in  favour  of  a  materialistic  theory.  We  have  here  a 
good  illustration  of  the  fact  that  the  path  of  science  is  not 
always  in  a  forward  direction  —  not  alwaj^s  like  the  march  of 
an  army  toward  some  definite  end.  Says  Langley,  "  I  believe 
this  comparison  of  the  progress  of  science  to  that  of  an  army, 
which  obeys  an  impulse  from  one  head,  has  more  error  than 

1  Thurston,  op.  cit.,  p.  83.  Then,  and  for  a  long  time  after,  the  study 
of  heat  was  taken  up  in  chemistry,  not  in  physics. 


iicixii    vvcvo   Kxivcii   lip  lii  Vyucjj-i 

2  Thurstox,  op.  cit.,  p.  87. 


114  A   HISTORY    OF   PHYSICS 

truth  in  it ;  and,  though,  all  similes  are  more  or  less  mislead- 
ing, I  would  prefer  to  ask  you  to  think  rather  of  a  moving 
crowd.,  where  the  direction  of  the  whole  comes  somehow  from 
the  independent  impulses  of  its  individual  members;  not 
wholly  unlike  a  pack  of  hounds,  which,  in  the  long  run,  per- 
haps, catches  its  game,  but  where,  nevertheless,  when  at  fault, 
each  individual  goes  his  own  way,  by  scent,  not  by  sight,  some 
running  back  and  some  forward ;  where  the  louder-voiced  bring 
many  to  follow  them,  nearly  as  often  in  the  wrong  path  as  in 
the  right  one ;  where  the  entire  pack  even  has  been  known 
to  move  off  bodily  on  a  false  scent.''  ^ 

The  earliest  traces  of  the  theory  that  heat  is  matter  are 
found  in  ancient  Greece  among  Democritus  and  Epicurus.  In 
modern  times  it  was  advocated  by  Pierre  Gassendi  (1592-1655), 
who  was  at  one  time  professor  of  mathematics  at  the  ColUge 
Royal  in  Paris.  He  was  a  man  of  ability,  but  in  physics 
his  efforts  were  speculative  rather  than  experimental.^  The 
acceptance  of  the  theory  that  heat  is  a  material  agent  was 
facilitated  through  the  previous  introduction  by  Georg  Ernst 
Stahl  (1660-1734),  professor  at  the  University  of  Halle,  of  the 
erroneous  theory  of  combustion,  according  to  which  a  burning 
body  gave  off  a  substance  called  "  phlogiston."  One  such  agent 
paved  the  way  for  the  other.  In  1738  the  French  Academy 
of  Sciences  offered  a  prize  question  on  the  nature  of  heat. 
The  winners  of  the  prize  (Euler  was  one  of  the  three)  favoured 
the  materialistic  theory.^  At  first  the  only  properties  postu- 
lated for  this  material  agent,  called  heat,  were  that  it  was 
highly  elastic  and  that  its  particles  repelled  each  other.  By 
this  repulsion  the  fact  that  hot  bodies  give  off  heat  could  be 

1  S.  P.  Langley,  op.  cit.,  p.  2. 

2  G.  Berthold,  Rumford  und  d.  Mech.  Wdrmetheorie,  Heidelberg, 
1875,  pp.  2-5. 

8  Berthold,  op.  cit.,  p.  6. 


HEAT  115 

explained.  Later  it  was  assumed  that  tlie  heat  particles  at 
tracted  ordinary  matter,  and  that  this  heat  was  distributed 
among  bodies  in  quantities  proportional  to  their  mutual  at- 
tractions (or  their  capacities  for  heat).  By  the  close  of  the 
eighteenth  century  this  theory  met  with  almost  universal 
acceptance.  Marat,  afterwards  famous  as  a  leader  in  the 
French  Eevolution,  gave  in  1780  an  exposition  of  this  theory 
by  starting  from  Newton's  corpuscular  theory  of  light.  It 
was  first  vigorously  attacked  by  an  American,  Count  Eumford, 
but  as  late  as  1856  it  received  preference  over  the  dynamic 
theory  in  the  article  "  Heat "  in  the  Encydopcedia  Britannica 
(8th  edition). 

In  spite  of  erroneous  theories  some  new  facts  were  found 
out  regarding  heat.  Black  discovered  what  he  termed  "  latent 
heat"  and  "capacity  for  heat"  (specific  heat).  Joseph  Black 
(1728-1799)  was  born  at  Bordeaux,  where  his  father,  a  native 
of  Belfast,  was  settled  as  a  wine  merchant.  He  was  professor 
at  Glasgow,  and  at  Edinburgh  after  1766.  He  is  well  known 
as  the  founder  of  pneumatic  chemistry. 

In  1756  he  began  to  meditate  over  the  perplexing  slowness 
with  which  ice  melts  and  water  is  dissipated  in  boiling.  He 
finally  concluded  that  a  large  quantity  of  heat  is  consumed 
simply  in  bringing  about  these  changes  of  state,  without  even 
the  least  alteration  in  temperature,  and  that  the  cause  of  this 
disappearance  is  a  quasi-chemical  combination  between  the 
particles  of  the  substance  and  the  subtle  fluid  called  heat. 
According  to  his  view  this  heat  was  "latent";  according  to 
the  modern  view  there  is  no  "  latent  heat,"  but  a  transforma- 
tion of  energy  takes  place,  the  energy  in  form  of  heat  becom- 
ing potential  energy  conferred  on  the  material  particles.-^ 
Modern  students  need  not  feel  disheartened  over  their  failure 

1  Die.  of  Nat.  Biog. 


116  A   HISTORY   OF  PHYSICS 

to  obtain  at  once  accurate  values  for  the  heat  of  vaporization 
of  water.  The  famous  Black  and  his  pupil,  Irvine,  obtained 
417,  later  450 ;  the  true  value  (at  standard  atmospheric  press- 
ure) being  536.  For  the  heat  of  fusion  he  obtained  by  the 
method  of  mixtures  77.8,  the  more  accurate  value  being  80.03 
(Bunsen). 

During  Black's  lifetime  his  great  discoveries  on  heat  re- 
mained unpublished,  but  after  1761  he  explained  them  in  his 
lectures,  dwelling  with  sedate  eloquence  on  the  beneficent 
effects  of  the  arrangement  in  checking  and  regulating  the 
processes  of  nature.^  His  discoveries  not  only  formed  the 
basis  of  calorimetry,  but  they  gave  the  first  impulse  to  Watt's 
improvements  in  the  steam-engine. 

Disliking  the  publicity  of  authorship.  Black  did  not  vindi- 
cate his  claims  to  priority.  As  might  be  expected,  the  same 
ideas  were  worked  out  by  others.  Jean  Andre  Deluc  in  Paris 
and  JoJiann  Karl  WilJce  in  Sweden  worked  along  the  same 
lines. 

The  great  chemist,  Antoine  Laurent  Lavoisier  (1743-1794), 
guillotined  during  the  French  K-evolution,  may  be  regarded 
as  a  disciple  of  Black.  In  conjunction  with  Pierre  Simon 
Laplace  (1749-1827),  Lavoisier  determined,  about  1783,  the 
specific  heats  of  a  number  of  substances.  They  designed  the 
instrument  now  known  as  Laplace's  ice  calorimeter,  but  Black 
and  Wilke  had  employed  the  method  of  the  ice  calorimeter 
before  them.^ 

1  Die.  Nat.  Biog. 

2  Lavoisier  and  Laplace's  joint  papers  appear  in  Memoires  de  VAcade- 
mie,  1780,  p.  355  (actually  printed  three  or  four  years  after  this  date). 
They  are  reprinted  in  German  translation  in  Ostwald''s  Klassiker,  No.  40. 
The  reference  to  Wilke  is  on  p.  22  of  this  reprint. 


ELECTRICITY   AND  MAGNETISM  117 

ELECTEICITY  AND  MAGNETISM 

No  branch  of  physics  was  cultivated  during  the  eighteenth 
century  so  successfully  as  electricity.  Eesearch  was  confined 
to  electro-statics  until  about  1790,  when  the  study  of  current 
electricity  began. 

Stephen  Gray  (?-1736),  a  pensioner  at  the  Charterhouse, 
England,  discovered  that  the  difference  in  electric  conduc- 
tivity depends,  not  upon  the  colour  of  objects  or  some  similar 
quality,  but  on  the  material  of  which  bodies  are  composed. 
Thus,  metal  wire  conducts ;  silk  does  not.  He  demonstrated 
that  the  human  body  is  a  conductor,  and  was  the  first  to 
electrify  a  human  being  (1730).  A  boy  was  suspended  in 
the  air  by  silken  strings.  Later  Gray  observed  that  con- 
ductors can  be  insulated  by  placing  them  on  cakes  of  resin. 

In  France,  Gray's  experiments  attracted  the  attention  of 
Charles  Frangois  de  Cisternay  du  Fay  (1698-1739),  who  had 
been  educated  as  a  soldier  but  devoted  his  maturer  years  to 
scientific  pursuits.  Experimentation  led  him  to  the  unex- 
pected conclusion  that  all  bodies  admit  of  being  electrified; 
in  other  words,  that  all  bodies  possess  the  property  which 
for  ages  was  supposed  to  be  peculiar  to  amber.  Hence  the 
classification  of  bodies  (introduced  by  Gilbert)  into  "electrics" 
(capable  of  being  electrified  by  friction)  and  "non-electrics" 
(not  possessing  this  property)  was  found  to  have  no  founda- 
tion in  fact.  Du  Fay  noticed  the  discharging  power  of  flames. 
Suspending  himself  by  silk  cords,  in  the  manner  taught  by 
Gray,  he  observed  that  when  he  was  electrified  and  another 
person  came  near,  there  issued  from  his  body  pricking  shoots, 
making  a  crackling  noise.  In  the  dark  these  shoots  were  so 
many  sparks  of  fire.  "The  Abbe  Nollet  says  he  shall  never 
forget  the  surprise  which  the  first  electrical  spark  which  was 


118  A   HISTORY    OF   PHYSICS 

ever  drawn  from  the  human  body  excited,  both  in  Mr.  Du  Fay 
and  in  himself."  ^ 

Du  Fay  discovered  that  there  are  two  kinds  of  electricity, 
which  he  named  the  vitreous  and  the  resinous.  Later  the 
same  observation  was  made  independently  by  Ehenezer  Kin- 
nersley  of  Philadelphia.  To  explain  electric  attraction  and 
repulsion  Du  Fay  postulated  the  existence  of  two  fluids  which 
are  separated  by  friction  and  which  neutralize  each  other  when 
they  combine.  This  is  the  earliest  important  attempt  at  a 
theory  of  electric  phenomena.  It  was  elaborated  more  fully 
as  a  rival  of  Franklin's  one-fluid  theory  by  the  Englishman 
Robert  Symmer. 

Considerable  attention  was  paid  at  this  time  to  the  perfec- 
tion of  the  electric  friction  machine.  It  assumed  a  supreme 
importance  in  laboratories,  until  finally  it  was  supplanted  by 
the  influence  machines  of  Holtz  and  Topler.  For  Hauksbee's 
glass  globe,  Andrew  Gordon  in  Erfurt  substituted  a  glass  cyl- 
inder. Martin  Planta  of  Grison,  Switzerland,  and  later  the 
optician  Jesse  Bamsden,  of  London,  introduced  circular  glass 
plates.  In  place  of  the  dry  palm  of  the  hand,  held  against  the 
rotating  glass,  Johann  Heinrich  Winkler  of  Leipzig  prepared  a 
leather  cushion  rubber,  which  was  pressed  against  the  glass  by 
a  spring.  JoJin  Canton,  in  1762,  secured  still  better  results  by 
applying  tin  amalgam  to  the  rubber.^ 

About  1745  electric  experimentation  became  so  popular  that 
in  Holland  and  Germany  public  exhibitions  were  given.  Many 
persons  experimented  for  their  own  amusement.  Among  these 
was  Ewald  Georg  von  Kleist  (died  1748),  dean  of  the  cathedral 
in  Camin,  Pomerania.  Once,  in  1745,  he  endeavoured  to  charge 
a  bottle  by  conduction.     He  observed  that,  when  he  held  in 

1  Pkiestlet,  Hist,  of  Elect. ^  London,  1775,  p.  47. 

2  For  drawings  of  various  machines,  see  G.  Albrecht,  Gesch.  d.  Eleo 
tricitdt,  1885,  pp.  20-30  ;  Priestley,  Hist,  of  Elect,,  Plates  IV.-VIIL 


ELECTRICITY    AND    MAGNETISM  119 

Ms  hand  a  small  phial  with  a  nail  in  it  and  electrified  the 
nail  by  contact  with  the  conductor  of  a  machine,  the  nail 
became  so  strongly  electrified  that  by  touching  it  with  the 
other  hand  he  received  a  shock  which  stunned  his  arms  and 
shoulders.  The  same  discovery  was  made  in  1746  in  a  simi- 
lar manner  at  Leyden,  Holland.  Pieter  van  MusschenbroeJc 
(1692-1761),  in  his  day  a  renowned  physicist,  attempted  to 
electrify  water  in  a  bottle.  At  a  trial,  Ciinaeus,  one  of  his 
friends,  held  the  bottle  in  one  hand,  and  after  a  while  pro- 
ceeded with  the  other  hand  to  remove  the  wire  connecting  the 
water  to  the  prime  conductor.  He  was  surprised  by  a  sudden 
shock  in  his  arms  and  breast.  Thus  was  discovered  what 
we  now  call  the  "Leyden  jar."^  Musschenbroek  repeated  the 
experiment  and  then  wrote  to  E-eaumur  "that  he  would  not 
take  another  shock  for  the  kingdom  of  France."  More  heroic 
sentiments  were  expressed  by  Professor  Bose  of  Wittenberg. 
He  wished  he  might  die  by  the  electric  shock,  that  the  account 
of  his  death  might  furnish  an  article  for  the  memoirs  of  the 
French  Academy  of  Sciences.^ 

The  invention  of  the  Ley  den  jar  gave  still  greater  4dat  to 
electricity.  In  almost  every  country  of  Europe  numbers  of 
persons  gained  a  livelihood  by*  going  about  and  showing  the 
experiments.  Winkler  of  Leipzig  proved  that  Yon  Kleist  was 
wrong  in  supposing  that  the  human  body  played  an  essential 
part  in  the  discharge  of  the  jar.  He  pointed  out  that  any 
conductor  connecting  the  inside  coating  to  the  outside  fully 
answered  the  j)nrpose. 

Musschenbroek' s  letter  to  Eeaumur  did  not  deter  French 
philosophers    from    experimentation.      Abbe   NoUet,   who    in 

1  According  to  another  account,  Cunaeus  made  this  discovery  at  his 
home,  while  endeavouring  to  repeat  some  experiments  he  had  seen  Mus- 
schenbroek perform. 

2  Priestley,  op.  cit. ,  p.  86. 


120  A   HISTORY   OF   PHYSICS 

France  was  even  more  celebrated  than  was  Musschenbroek  in 
Holland,  repeated  the  Ley  den  jar  experiments  on  himself. 
He  then,  in  the  King's  presence,  passed  the  discharge  through 
180  guards.  Later  the  Carthusian  monks  at  the  convent  in 
Paris  were  formed  into  a  line  900  feet  long,  by  means  of  iron 
wires  between  every  two  persons,  and  the  whole  company, 
upon  the  discharge  of  the  jar,  gave  a  sudden  spring  at  the 
same  instant.  This  behaviour  of  the  austere  monks  must  have 
been  ludicrous  in  the  extreme.  Experimenters  in  France  and 
elsewhere  killed  birds  and  other  animals  by  the  discharge  of 
the  Ley  den  jar;  they  passed  the  discharge  long  distances 
through  water  across  rivers  and  lakes;  they  magnetized 
needles  by  it  and  melted  thin  wire.  The  discovery  of  the 
Leyden  jar  was  hailed  as  a  great  advance  in  science.  No 
doubt  its  importance  was  at  the  time  overestimated. 

Some  of  the  boldest  researches  and  profoundest  theories  of 
the  eighteenth  century  were  soon  to  be  advanced  in  far-off 
America  by  Benjamin  Franklin  (1706-1790).  Although  in  his 
youth  only  a  printer's  apprentice,  he  developed  into  a  man  of 
unusual  powers,  not  only  in  the  fields  of  politics  and  diplo- 
macy, but  also  in  physical  research.  At  the  age  of  forty  he 
happened  to  see  Dr.  Spence  from  Scotland  perform  some 
electrical  experiments  at  Boston.  The  subject  was  new  to 
him.  After  returning  to  Philadelphia  the  Library  Company 
in  that  city  received  from  Peter  CoIUnson,  a  London  merchant 
and  member  of  the  Eoyal  Society,  a  glass  tube,  with  instruc- 
tions how  to  use  it  in  electrical  experiments.  Franklin's 
curiosity  having  been  excited,  he  began  to  read  Watson's  ex- 
periments, and  also  to  experiment  for  himself.-^     In  his  first 

1  Works  of  Bevjamin  FranJcUn,  edited  by  Ja red  Sparks,  Boston,  1837, 
Vol.  v.,  pp.  173-180.  This  volume  contains  Franklin's  famous  letters  on 
electricity  ;  also  an  appendix  containing  letters  by  various  scientific  men 
respecting  Eranklin's  discoveries. 


ELECTKICITY   A^'D   :MAG^'ETISM  121 

letter  to  Collinson,  March.  28 j  1747,  he  expresses  thanks  for 
the  "  electric  tube/"'  and  savs :  "  I  never  was  before  engaged 
in  any  study  that  so  totally  engaged  my  attention  and  my 
time  as  this  has  lately  done."  ^  His  home  came  to  be  fre- 
quented by  curiosity  seekers.  There  was  formed  a  small 
group  of  investigators,  consisting  of  Franklin,  Ehenezer  Kin- 
nersley,  Tliomas  Hopkinson,  and  Philip  Sing.  In  the  next 
letter  to  Collinson,  July  11,  1717,  Franklin  describes  the 
"  wonderful  effect  of  pointed  bodies,  both  in  drawing  off  and 
throwing  off  the  electrical  fire."  This  action  of  points  had 
been  observed  by  others,  but  Franklin  was  the  first  fully  to 
realize  its  importance  and  to  put  it  to  use. 

This  same  letter  contains  Franklin's  theory  of  electricity, 
which,  explained  phenomena  more  satisfactorily  than  any  other 
proposed  up  to  that  time.  He  supposed  that  "'electric  fire  is 
a  common  element,"  existing  in  all  bodies.  If  a  body  acquired 
more  than  its  normal  share,  it  was  called  "plus  " ;  if  less,  it  was 
designated  '^  minus."  T-hus,  instead  of  Du  Fay's  two-fluid 
theory,  Franklin  advocated  a  one-fluid  theory.  To  him  we 
owe  the  terms  "'plus"  and  "minus,"  or  "positive"  and  "nega- 
tive "  electricity.  This  material  theory  held  its  own  until  the 
times  of  Faraday  and  Maxwell.  Since  then  we  have  become 
quite  convinced  that  electricity  is  not  matter.  Franklin  ex- 
plained the  charged  Leyden  jar  as  containing  on  one  coating  an 
excess  of  the  fluid,  "  a  plenum  of  electrical  fire,"  and  on  the 
other  a  "vacuum  of  the  same  fire,"  but  really  containing  no 
more  electricity  than  before  charging.-  He  showed  experi- 
mentally that  "the  whole  force  of  the  bottle  and  power  of 
giving  a  shock  is  in  the  glass  itself"  (p.  201). 

In  1718  Franklin  sold  his  printing  house,  newspaper,  and 
almanac,  with  the  view  of  retiring  from  business  and  devoting 

-  Works,  Vol.  v.,  p.  181.  ^Ibidem,  p.  191. 


122  A   HISTORY    OF   PHYSICS 

all  his  time  to  electrical  experiments.  He  equipped  himself 
with  new  apparatus.  His  friend  Kinnersley  proved  that  the 
Leyden  phial  can  be  as  easily  electrified  by  sparks  passing  to 
the  outside  as  to  the  inside  (p.  197).  In  1749  Franklin  states 
in  a  letter  to  Collinson  that  "  hot  weather  is  coming  on,  when 
electrical  experiments  are  not  so  agreeable,"  and  he  proposed 
to  end  the  season  with  an  electric  party.  "  A  turkey  is  to  be 
killed  for  our  dinner  by  the  electrical  shock,  and  roasted  by  the 
electrical  jack,  before  a  fire  kindled  by  the  electrified  bottle  " 
(p.  211).  But  before  the  summer  of  1749  he  entered  upon 
more  serious  reflections.  At  this  time  Franklin  first  suggested 
the  idea  of  explaining  lightning  on  electrical  principles.  The 
conjecture  that  the  nature  of  lightning  was  the  same  as  that  of 
the  electric  spark  had  been  made  before.  Gray,  Wall,  Nollet, 
Freke,  Winkler,  had  all  expressed  this  thought.^  Franklin 
probably  did  not  know  of  these  conjectures.  Though  contrary 
to  the  then  prevalent  theory  of  lightning,  they  certainly  war- 
ranted some  one  in  making  an  experimental  test.  Thunder 
and  lightning  were  generally  believed  to  be  due  to  exploding 
gases,  though  opinions  differed  as  to  the  nature  of  the  gases. 
In  1737  Franklin  believed  lightning  to  be  due  to  "the  inflam- 
mable breath  of  the  Pyrites,  which  is  a  subtle  sulphur,  and 
takes  fire  of  itself."  As  already  stated,  in  the  early  summer 
of  1749,  he  advanced  the  electrical  theory,  and  conceived  bold 
plans  for  experimentation.  The  heat  of  summer  did  not  deter 
him  and  Kinnersley  from  experimentation.  Under  the  date  of 
November  7,  1749,  the  following  passage  is  found  in  his  note- 
book: "Electrical  fluid  agrees  with  lightning  in  these  par- 
ticulars :  (1)  Giving  light  •,  (2)  colour  of  the  light ;  (3)  crooked 
direction ;  (4)  swift  motion ;  (5)  being  conducted  by  metals  ; 
(6)  crack  or  noise  in   exploding ;    (7)  subsisting  in  water  or 

*  Benjamin,  p.  575, 


ELECTRICITY   AND    MAGNETISM 


123 


ice;  (8)  rending  bodies  it  passes  througli;  (9)  destroying 
animals;  (10)  melting  metals;  (11)  firing  inflammable  sub- 
stances ;  (12)  sulphurous  smell."  Will  lightning  be  attracted 
and  drawn  off  by  points  like  the  electric  fluid  in  his  jars? 
"  Since  they  agree  in  all  the  particulars  wherein  we  can  already 
compare  them,  is  it  not  probable  that  they  agree  likewise  in 
this  ?  Let  the  experiment  be  made."  By  the  action  of  points 
he  proposed  to  draw  down  the  lightning.  "  On  the  top  of  some 
high  tower  or  steeple,  place  a  kind  of  sentry-box  (as  in  Pig. 
13),  big  enough  to  contain  a  man  and  an  elec- 
trical stand.  From  the  middle  of  the  stand  let 
an  iron  rod  rise  and  pass,  bending  out  of  the 
door,  and  then  upright  twenty  or  thirty  feet, 
pointed  very  sharp  at  the  end.  If  the  electrical 
stand  be  kept  clean  and  dry,  a  man  standing  on 
it,  when  such  clouds  are  passing  low,  might  be 
electrified  and  afford  sparks,  the  rod  drawing 
fire  to  him  from  a  cloud."  "If  these  things 
are  so,  may  not  the  knowledge  of  this  power 
of  points  be  of  use  to  mankind  in  preserv- 
ing houses,  churches,  ships,  etc.,  from  the  stroke  of  light- 
ning? ...  "V. 

Such  are  the  thoughts  communicated  in  a  letter  to  Collinson 
in  July,  1750,  and  submitted  by  him  to  the  Eoyal  Society. 
That  body  at  first  received  the  new  ideas  with  derision.  The 
plans  seemed  visionary.^  As  the  Eoyal  Society  failed  to  pub- 
lish anything  but  a  brief  notice  of  Franklin's  researches,  Col- 


FiG.  13. 


1  Works,  Vol.  v.,  pp.  236,  237. 

2  Three  years  later  (1753),  after  Franklin's  researches  had  met  with 
enthusiastic  appreciation  on  the  part  of  French  scientists  and  the  French 
king,  the  Royal  Society  awarded  him  the  Copley  medal.  The  president's 
address  on  the  occasion  of  the  award  is  given  in  Franklin's  Works,  Vol,  V., 
pp.  499-504.    In  1756  Franklin  was  elected  member  of  the  Royal  Society. 


124  A   HISTORY    OF   PHYSICS 

linson  determined  to  bring  out  the  letters  without  its  imprint. 
By  the  additional  letters  that  arrived  later,  they  swelled  to  a 
quarto  volume  which  passed  through  five  editions.  Seventeen 
years  after  the  first  xjublication,  Priestley  wrote :  "  Nothing 
was  ever  written  upon  the  subject  of  electricity  which  was 
more  generally  read  and  admired  in  all  parts  of  Europe  than 
these  letters.  There  is  hardly  any  European  language  into 
which  they  have  not  been  translated ;  and,  as  if  this  were 
not  sufficient  to  make  them  properly  known,  a  translation  of 
them  has  lately  been  made  in  Latin."  ^ 

In  America  popular  curiosity  ran  high.  Kinnersley  started 
on  a  lecturing  tour,  showing  electric  experiments  and  winning 
applause.  In  New  York,  Newport,  and  Boston  these  lectures 
produced  a  genuine  sensation.  "  Eaneuil  Hall  resounded  with 
the  cracks  and  snaps  of  his  jars  and  globes,  long  before  they 
echoed  the  impassioned  eloquence  of  the  orators  of  the  devo- 
lution." ^ 

Franklin  was  of  the  opinion  that  no  building  in  Philadelphia 
or  hill  near  by  was  high  enough  to  enable  him  to  perform  the 
experiment  with  the  sentry-box.  While  he  was  endeavouring 
to  raise  money  by  means  of  a  lottery  for  the  erection  of  a  spire 
of  sufficient  height,  news  came  that  the  experiment  had  been 
tried  successfully  at  Marly-la- Ville,  near  Paris,  by  Dalibard, 
under  the  auspices  of  the  Erench  king.  How  was  it  done  ? 
Simply  by  a  rod  13  metres  (40  feet)  high,  insulated  at  its 
base,  and  resting  upon  a  small  table  within  a  small  cabin. 
Dalibard  instructed  an  old  dragoon  to  watch  for  clouds.  A 
brass  wire  mounted  in  a  glass  bottle  was  gotten  ready  for  the 
purpose  of  drawing  off  sparks  from  the  rod.  After  several 
days'  waiting,  a  thunder-cloud  appeared  on  May  10, 1752.  The 
dragoon  approached  the  wire  to  the  rod,  and  there  was  a  lively 

1  Pkiestley,  Hist,  of  Elect.,  p.  154.  ^  Benjamin,  p.  585. 


ELECTRICITY    AND   MAGNETISM  125 

crackling  of  sparks.  The  flame  and  sulphurous  odour  were 
evidently  infernal.  The  terrified  dragoon  dropped  the  wire 
and  shouted  to  his  neighbours  to  send  for  the  village  priest. 
The  latter  was  braver  than  the  dragoon.  He  began  to  experi- 
ment for  himself  and  drew  sparks  from  the  rod.  He  com- 
municated the  results  to  Dalibard.^  "Franklin's  idea  ceases 
to  be  a  conjecture/'  writes  Dalibard;  "here  it  has  become  a 
reality."  A  week  later  Delor  in  Paris  repeated  the  experi- 
ment with  a  rod  32  metres  (99  feet)  high. 

Franklin  himself  did  not  regard  the  tests  in  Paris  con- 
clusive. He  was  not  fully  convinced  that  the  Frenchmen's 
rods  had  become  electrified  by  lightning.  The  rods  did  not 
reach  up  into  the  clouds.  A  new  idea  flashed  into  his  mind. 
AVhy  not  send  a  kite  up  into  the  very  interior  of  the  cloud, 
and  conduct  the  lightning  down  on  its  cord  ?  He  prepared  a 
kite.  "  Make  a  small  cross,"  he  writes  afterwards  to  Collinson, 
"  of  two  light  strips  of  cedar,  the  arms  so  long  as  to  reach  to 
the  four  corners  of  a  large  thin  silk  handkerchief  when  ex- 
tended ;  tie  the  corners  of  the  handkerchief  to  the  extremities 
of  the  cross,  so  you  have  the  body  of  a  kite ;  ...  to  the  top  of 
the  upright  stick  of  the  cross  is  to  be  fixed  a  very  sharp- 
pointed  wire,  rising  a  foot  or  more  above  the  wood.  To  the 
end  of  the  twine  next  the  hand  is  to  be  tied  a  silk  ribbon,  and 
where  the  silk  and  twine  join,  a  key  may  be  fastened."  ^  With 
this  apparatus  he  went  out  on  the  common,  accompanied  only 
by  his  son.  He  placed  himself  under  a  shed  to  avoid  the  rain, 
and  raised  the  kite.  A  thunder-cloud  passed,  but  as  yet  there 
was  no  sign  of  electricity.  He  almost  despaired  of  success, 
when  suddenly  he  observed  the  loose  fibres  of  the  string  erect 

1  The  priest's  letter,  as  also  Dalibard's  communication  to  the  French 
Academy,  are  given  in  Franklin's  Works^  Vol.  V. ,  pp.  288-293.  See  also 
Benjamin,  p.  588. 

2  Franklin's  Works,  Vol.  V.,  p.  295. 


126  A    HISTORY    OF   PHYSICS 

themselves.  He  now  presented  a  knuckle  to  the  key,  and  re- 
ceived a  strong  spark.^  What  exquisite  pleasure  that  spark 
must  have  given  him  !  More  sparks  were  obtained ;  a  Leyden 
jar  was  charged,  a  shock  given,  etc.  He  had  demonstrated 
that  lightning  is  an  electric  phenomenon. 

"  In  September,  1752,"  says  Franklin,  "  I  erected  an  iron  rod 
to  draw  the  lightning  down  into  my  house,  in  order  to  make 
some  experiments  on  it,  with  two  bells  to  give  notice  when  the 
rod  should  be  electrified."  -  He  then  concluded  from  a  number 
of  experiments  "  that  the  clouds  of  thunder-gust  are  most  com- 
monly in  a  negative  state  of  electricity,  but  sometimes  in  a 
positive  state"  (p.  304).^  Hence,  "for  the  most  part,  in 
thunder-strokes  it  is  the  earth  that  strikes  into  the  clouds,  and 
not  the  clouds  that  strike  into  the  earth  "  (p.  305). 

Franklin's  experiments  on  atmospheric  electricity  were  re- 
peated everywhere.  The  French  physician,  Louis  Guillaume 
Lemonnier,  found  that  the  atmosphere  is  always  electric,  even 
when  no  clouds  are  in  sight.  Georg  Wilhelm  Richmann,  of 
St.  Petersburg,  was  struck  dead  while  experimenting  with 
lightning  in  1753.  Detailed  reports  of  the  effect  on  the 
various  organs  of  his  body  were  published  by  scientific  socie- 
ties. Says  Priestley,^  "  It  is  not  given  to  every  electrician  to 
die  in  so  glorious  a  manner  as  the  justly  envied  Eichmann." 

Franklin's  suggestion  to  protect  buildings  by  lightning-rods 
was  first  carried  out  in  1754  by  Procopius  DiviscJi,  a  clergyman 
at  Prenditz,  in  Mahren.  In  1760  Franklin  erected  one  on  a 
building  in  Philadelphia.  WilUa^n  Watso7i  erected  the  first 
lightning-rod  in  England  in  1762.     In  1782  there  were  about 

1  Franklin's  Works,  Vol.  Y.,  p.  175. 

2  Ibidem,  p.  301. 

3  That  atmospheric  electricity  may  vary  in  sign  had  been  noticed 
before  this  by  John  Canton. 

■^  Hist,  of  Elect.,  p.  86. 


ELECTRICITY   AND   MAGNETISM  127 

400  rods  in  Philadelpliia.  At  first  some  opposition  to  theii 
erection  was  made  by  certain  theologians.  It  was  argued  that 
as  thunder  and  lightning  were  tokens  of  divine  wrath,  it  was 
impious  to  interfere  with  their  power  of  destruction.^  To 
this  argument  John  WintJirop,  the  first  professor  of  physics 
at  Harvard  College,  gave  the  common-sense  reply :  "  It  is  as 
much  our  duty  to  secure  ourselves  against  the  effects  of  light- 
ning as  against  those  of  rain,  snow,  and  wind,  by  the  means 
God  has  i)ut  into  our  hands."  ^ 

Experience  soon  proved  that  rods  were  not  an  absolute  pro- 
tection against  lightning.  Failure  to  protect  was  then  and 
long  afterwards  attributed  either  to  bad  earth  connection  or 
to  dull  points.  Various  improvements  in  construction  were 
suggested.^  But  the  real  difficulty  was  not  recognized  until 
nearly  a  century  later.  Franklin's  theory  of  the  action  of 
the  rod  was  incomplete.  We  now  begin  to  see  that  the  fail- 
ure of  carefully  erected  rods  to  protect  is  due  to  the  fact 
that  the  discharge  may  be  oscillatory.'* 

1  A.  D.  White,  Warfare  of  Science  with  Theology,  1896,  Vol.  I., 
p.  366. 

2  For  an  account  of  John  Winthrop,  see  "W.  J.  Youmans,  Pioneers  of 
Science  in  America,  New  York,  1896. 

3  See,  for  instance,  a  paper  by  Robert  Patterson  of  Philadelphia  in 
Trans.  Am.  Philos.  Society,  Vol.  III.,  1793,  pp.  122,  321. 

*  Franklin  supplied  Harvard  College  with  electrical  apparatus.  In  a 
letter  of  1753  he  speaks  of  the  shipment  of  Leyden  jars.  Before  this  time 
the  instruction  in  electricity  at  Harvard  must  have  been  quite  meagre. 
Among  manuscript  notes  of  excellent  lectures  on  astronomy,  and  a  few  on 
light  and  electricity,  prepared  by  John  Winthrop  in  1750,  there  is  only 
one  lecture  on  electricity  and  magnetism.  Trowbridge  gives  part  of  these 
notes  as  follows  :  "  If  a  flaxen  string  be  extended  and  supported,  and  at 
one  end  an  excited  tube  be  applied,  light  bodies  will  be  attracted,  and 
that  at  the  distance  of  1200  feet  at  the  other  end.  This  electricity  since 
the  year  1743  has  made  a  considerable  noise  in  the  world,  upon  which 
it  is  supposed  several  of  the  (at  present)  hidden  phenomena  of  nature 
depend.  .  .  .     Men  have  been  so  electrized  as  to  have  considerable  light 


128  A    HISTORY    OF   PHYSICS 

In  1703  Dutch  travellers  brouglit  tourmaline  from  Ceylon. 
They  observed  that  it  was  capable  of  attracting  the  light  ashes 
on  glowing  peat.  Its  properties  were  examined  by  Franz 
Ulrich  Theoclor  u^pinus  and  Johann  Karl  Wilke,  who  con- 
cluded that  it  became  electrified  by  heating,  its  ends  carrying 
charges  of  opposite  sign.  Torbern  Olof  Bergman  showed  in 
1766  that  it  was  not  so  much  the  heat  that  produced  electricity 
as  it  was  the  difference  in  temperature  between  its  parts ;  that 
on  cooling  the  charge  at  each  end  is  reversed.  Beiijamin 
Wilson  and  John  Canton  found  that  the  electric  property  of 
tourmaline  was  shared  by  other  crystals. 

During  the  latter  part  of  the  eighteenth  century  the  fi.rst 
important  steps  were  taken  in  the  way  of  exact  measurements 
in  static  electricity.  In  this  field  of  research  we  meet  two  great 
names,  Cavendish  and  Coulomb.  Henry  Cavendish'^  (1731- 
1810)  attended  Peterhouse  College,  Cambridge,  and  afterward 
lived  chiefly  in  London.  The  great  obscurity  hanging  over 
his  private  history  has  rendered  it  impossible  to  ascertain 
what  influences  induced  him  to  devote  himself  to  experimental 
science.  He  experimented  in  chemistry,  heat,  electricity,  but 
he  took  little  pains  to  publish  his  results  and  to  secure  priority 
of  discovery.  He  lived  a  strangely  retired  life.  Being  of 
frugal  habits,  he  allowed  his  large  income  to  accumulate.  "  He 
received  no  stranger  at  his  residence;  he  ordered  his  dinner 
daily  by  a  note  left  on  the  hall  table,  and  from  his  morbid 

round  their  heads  and  bodies,  not  unlike  the  light  represented  around  the 
heads  of  saints  by  painters."  Trowbridge  adds  that  "the  entire  appara- 
tus to  illustrate  the  subject  of  electricity  and  magnetism  in  Harvard  Uni- 
versity until  1820  consisted  merely  of  two  Franklin  electrical  machines, 
a  collection  of  Leyden  jars,  and  small  apparatus  to  illustrate  the  effects 
of  electrical  attractions  and  repulsions  shown  by  electrified  pith  balls  or 
similar  light  objects."  See  John  Trowbridge,  What  is  Electricity  9 
1897,  p.  26. 

1  Die.  Kat.  Biog. 


ELECTRICITY   AND   MAGNETISM  129 

shyness  lie  objected  to  any  communication  with  his  female 
domestics."^  "He  probably  uttered  fewer  words  in  the  course 
of  his  life  than  any  man  who  ever  lived  to  fourscore  years, 
not  at  all  excepting  the  monks  of  La  Trappe."  ^  Cavendish's 
whole  existence  was  in  his  laboratory  and  his  library.^  His 
experiments  on  electrostatics  were  completed  before  the  end 
of  the  year  1773,  but  remained  unpublished.  He  printed  only 
two  electric  papers,  and  these  contained  matter  of  secondary 
importance.  About  a  century  later,  in  1879,  James  Clerk 
Maxwell  published  a  book  under  the  title,  Electrical  Researches 
of  the  Honourable  Henry  Cavendish,  written  between  1771  and 
1781.  "  These  papers,"  says  Maxwell,  "  prove  that  Cavendish 
had  anticipated  nearly  all  those  great  facts  in  electricity  which 
at  a  later  period  were  made  known  to  the  scientific  world 
through  the  writings  of  Coulomb  and  the  French  philosophers." 
Cavendish  made  the  capacity  of  condensers  a  subject  of  inves- 
tigation, and  constructed  for  himself  a  complete  set  of  con- 
densers of  known  capacity,  by  which  he  measured  the  capacity 
of  various  pieces  of  apparatus.  A  battery  of  49  jars  was 
found  to  contain  321,000  "  inches  of  electricity  "  (about  i  micro- 
farad). His  "inches  of  electricity"  express  the  diameter  of 
the  sphere  of  equivalent  capacity.  Our  modern  electrostatic 
measurements  of  capacity  differ  from  this  simply  in  the  use 


1  Die.  Nat.  Biog. 

2  Lord  Brougham,  Lives  of  Philosophers.,  London,  1855,  p.  106. 

^  Henry  Cavendish  happened  once  at  a  dinner  to  sit  next  to  William 
Herschel,  who  had  "been  constructing  telescopes  of  such  unheard-of  mag- 
nitude and  accuracy  of  figure  that  a  star  could  be  seen  without  "rays." 
Cavendish  slowly  addressed  the  astronomer  with,  "  Is  it  true.  Dr.  Herschel, 
that  you  see  the  stars  round  ?  "  "  Round  as  a  button, ' '  exclaimed  the 
doctor,  when  the  conversation  dropped,  till  at  the  close  of  dinner,  Caven- 
dish repeated  interrogatively,  "Round  as  a  button?"  "Round  as  a 
button,"  briskly  rejoined  the  doctor,  and  no  more  was  said.  From  arti- 
cle "Herschel,  Sir  William,"  in  Die.  Nat.  Biog. 


130  A   HISTORY    OF   PHYSICS 

of  "  centimeters  "  and  "  radius "  in  place  of  "  inches "  and 
"  diameter."  Cavendish  anticipated  Faraday  in  the  discovery 
of  specific  inductive  capacity  of  different  substances,  and  meas- 
ured this  quantity  for  several  substances.  For  parafiB.n  he 
found  the  values  1.81  to  2.47,  while  more  recently  Boltzmann 
has  given  2.32,  Wlillner  1.96,  Gordon  1.994.^  The  preceding 
ideas  presuppose  the  notion  of  potential.  This  was  intro- 
duced by  Cavendish  under  the  name  "degree  of  electrifica- 
tion." He  proved  that  static  charges  reside  on  the  surfaces 
of  conductors  and  that  the  electric  force  varies  inversely  as 
the  square  of  the  distance,  or  at  least  cannot  differ  from  that 
ratio  by  more  than  Jq  part.  In  1781  he  completed  an  inquiry 
which  amounts  to  an  anticipation  of  Ohm's  Law.^ 

It  is  a  matter  of  regret  that  Cavendish  did  not  give  scientists 
of  his  day  the  benefit  of  his  far-reaching  results.  It  is  re- 
markable that,  while  Cavendish  originated  new  concepts,  and 
engaged  largely  in  electric  measurements,  he  was  no  inventor 
of  new  apparatus.  Coulomb  invented  the  torsion  electrometer ; 
Abraham  Bennet,  in  1786,  brought  forth  the  gold-leaf  electro- 
scope ;  but  Cavendish  designed  no  similar  instruments.  He 
used  the  pith-ball  electrometer. 

Charles  Augustin  Coulomb  (1736-1806)  was  born  at  Angou- 
l§me,  studied  in  Paris,  and  at  an  early  age  entered  the  army. 
After  several  years'  service  in  the  West  Indies,  he  returned  to 
Paris  and  served  as  engineer.  He  engaged  at  the  same  time 
in  scientific  research.  When  a  project  of  navigable  canals  in 
Bretagne  was  under  consideration.  Coulomb  was  appointed  by 
the  minister  of  marine  to  examine  the  ground.  His  report  was 
unfavourable.  This  displeased  some  influential  persons,  and 
under  the  pretext  that  he  had  no  orders  from  the  minister  of 

1  Maxwell,  Elect.  Besearches  of  the  Hon.  H.  Cavendish.,  p.  liii. 

2  Ibidem,  pp.  lix.,  §§  574,  575,  629,  686.  The  law  is  not  worked  out 
by  him  as  carefully  and  systematically  as  by  Ohm  over  forty  years  later. 


ELECTRICITY   AKD   MAGNETISM  131 

war,  they  placed  him  in  confinement.  Later  the  government 
of  Bretagne  saw  its  error,  and  offered  Coulomb  a  large  recom- 
pense, but  he  accepted  only  a  seconds  watch,  which  he  after- 
wards used  in  experimentation.  Says  Thomas  Young:  "His 
moral  character  is  said  to  have  been  as  correct  as  his  mathe- 
matical investigations."  ^ 

Coulomb  entered  upon  researches  on  the  torsional  elasticity 
of  hairs  and  wires.  This  led  in  1777  to  the  torsion  balance, 
or  "balance  de  torsion."  Some  similar  device  had  been  pre- 
viously suggested  in  England  by  John  Michell.^  The  torsion 
balance  has  held  its  place  in  texts  on  electricity  for  a  century, 
though  now  the  instrument  is  no  longer  used  in  the  laboratory. 
Coulomb  experimented  with  great  ingenuity  and  accuracy, 
and  proved  with  it  that  Newton's  law  of  inverse  squares  holds 
also  in  electric  and  in  magnetic  attraction  and  repulsion.^  He 
proved  that  the  action  varies  as  the  product  of  the  quantities 
of  electricity;  he  showed  also  that  electric  charges  exist  on 
the  surfaces  of  conductors  and  compared  the  surface  charges 
on  various  parts  of  a  conductor.  Coulomb  was  an  advocate  of 
the  two-fluid  theory  and  believed  that  attraction  and  repulsion 
take  place  by  an  "actio  in  distans,"  without  an  intervening 
medium.  His  electrical  memoirs,  which  appeared  between 
1785  and  1789,  furnished  the  data  on  which  Poisson  later 
founded  his  mathematical  theory  of  electricity.* 

Prom  very  early  times  it  was  known  that  certain  species  of 
water-animals  are  capable  of  giving  shocks.     After  the  inven- 

1  Misc.  Works,  Vol.  II.,  p.  540. 

2  Heller,  Vol.  II.,  p.  499. 

^  That  magnetic  action  follows  the  law  of  inverse  squares  was  shown 
before  this  (about  1760)  by  Tobias  Mayer  of  Gottingen.  See  Albeecht, 
Gesch.  cf  Elect.,  p.  75. 

*  Coulomb's  seven  papers  appeared  originally  in  the  Memoirs  de  VAco.- 
demie  royale  des  sciences,  1785  and  1786.  The  first  four  are  printed  in 
German  translation  in  Ostwald''s  Klassiker,  No.  13. 


132  A   HISTORY   OF   PHYSICS 

tion  of  the  Ley  den  jar,  men  began  to  ponder  over  the  similarity 
in  the  physiological  effect  of  its  discharge  to  that  of  shocks 
given  by  these  animals.  John  Walsh  made  the  first  thorough 
investigation  of  this  subject  at  La  E-ochelle  and  proved  that 
the  shocks  imparted  by  the  fishes  are  electrical.  Connecting 
the  back  and  under  side  of  the  fish  with  a  conductor,  a  dis- 
charge took  place.^ 

Among  those  interested  in  animal  electricity  was  Aloisio 
Galvani  (1737-1798),  a  physician  and  professor  of  anatomy  in 
Bologna.  By  accident  he  was  led  to  the  great  discovery  of 
current  electricity  or  "galvanism."  The  story  goes  that  his 
wife  was  in  poor  health  and  was  ordered  to  eat  frogs'  legs. 
Galvani  prepared  them  himself.  When  he  had  taken  off  their 
skins,  he  laid  them  on  a  table  near  the  conductor  of  a  charged 
electric  machine  and  left  the  room.  His  wife  chanced  to  hold 
the  scalpel  near  the  machine  while  at  the  same  time  the 
scalpel's  point  touched  the  exposed  crural  nerve  of  a  frog's 
leg.  A  spark  passed  and  the  leg  convulsed  violently.  She 
acquainted  her  husband  of  this,  and  he  repeated  the  experi- 
ment. This  occurred  on  November  6,  1780.  Galvani's  own 
account  is  more  prosaic.^  His  wife  plays  no  role  in  the  dis- 
covery ;  only  one  frog  is  dissected ;  an  assistant  first  notices 
the  twitching. 

Galvani  set  about  to  discover  the  cause.  It  seemed  neces- 
sary to  touch  a  nerve  and  to  have  a  spark.  The  effect  was  the 
same  when  the  legs  were  placed  in  a  vacuum.  The  question 
arose,  will  atmospheric  electricity  serve  as  well  as  that  from 
the  machine  ?  He  suspended  frogs'  legs  by  iron  hooks  from  an 
iron  trellis  in  the  garden.     The  legs  exhibited  motion.     It  was 

1  His  papers  appeared  in  the  Phil.  Trans,  for  1773  and  1774, 

2  See  Ostiimld's  Klassiker,  No.  52,  p.  4.  This  number  is  a  German 
translation  of  Galvani's  article,  "De  viribus  electricitatis  in  raoto  muscu' 
lari  commentarius,"  in  Comment.  Acad,  scient.  Bonoji.,  1791. 


ELECTKICITY   AND   MAGNETISM  133 

violent  when  storm-clouds  passed,  but  could  be  seen  at  times 
during  a  clear  sky.  At  first  lie  attributed  the  twitching  to 
changes  in  atmospheric  electricity.  He  abandoned  this  view 
after  he  succeeded  in  producing  the  same  effects  indoors  by 
placing  the  frogs'  legs  on  a  metallic  plate  and  allowing  the  wire 
piercing  the  crural  nerve  to  touch  the  plate.  The  cause  must 
lie  in  the  leg,  the  plate,  or  the  wire.  Galvani  placed  the  leg 
on  glass  and  touched  the  crural  nerve  and  a  muscle  of  the  foot, 
both  at  the  same  time,  with  the  ends  of  a  bent  rod.  If  the 
rod  was  of  glass,  no  effect  was  seen ;  if  it  was  of  copper  and 
iron,  or  copper  and  silver,  then  prolonged  convulsions  followed. 
The  fact  that  a  rod  of  iron  alone  produced  motion,  though  it 
was  not  so  continuous  and  pronounced  as  when  it  consisted  of 
two  metals,  led  Galvani  to  the  conclusion  that  the  rod  served 
merely  as  a  conductor.  Further  tests  seemed  to  him  to  locate 
the  source  of  electricity  in  the  nerve. 

Galvani's  observations  were  of  startling  novelty  and  aston- 
ished scientific  men  everywhere.  More  profound  than  his  own 
was  the  reasoning  on  this  subject  by  his  countryman,  Alessan- 
dro  Volta  (1745-1827),  who  occupied  the  chair  of  physics  for 
five  years  at  the  gymnasium  of  his  native  town,  Como,  and 
after  1779,  for  twenty-five  years,  the  chair  of  physics  at  the 
university  of  Pavia.  He  had  been  a  diligent  experimenter  in 
electricity  and  in  1775  had  invented  the  electrophorus.  He 
found  that  the  electric  discharge  through  a  nerve  could  pro- 
duce other  effects  than  motion.  If  a  bent  rod  of  two  metals 
touched  the  eye  above,  while  the  other  end  was  held  in  his 
mouth,  a  sensation  of  light  followed  at  the  moment  of  making 
contact.  A  silver  and  a  gold  coin  held  against  the  tongue 
produced  a  bitter  taste  as  soon  as  the  coins  were  connected  by 
a  wire.^     Thus,  the  electricity  was  able  not  only  to  produce 

1  This  bitter  taste  had  been  observed  previously  in  Germany  by  Johann 
Georg  Sulzer.     See  Edm,  Hoppe,  Gesch.  d.  Elect.,  Leipzig,  1884,  p.  128. 


134  A   HISTOHY    OF   PHYSICS 

motion,  but  to  affect  the  nerves  of  vision  and  taste.  Volta 
conjectured  that  tlie  essential  thing  in  all  these  experiments 
was  the  contact  of  different  metals.  After  1794  he  set  about 
to  prove  this  hypothesis.  If  Galvani  was  right  in  placing  the 
seat  of  electricity  in  the  frog's  leg  and  in  attributing  to  the 
metal  rod  merely  the  function  of  discharger,  as  in  the  Leyden 
jar,  then  one  metal  should  produce  twitching  as  easily  as  two. 
If  the  ends  of  a  wire  of  one  metal  are  at  different  temperar 
tures,  then  vigorous  convulsions  follow ;  they  disappear  almost 
entirely  on  equalization  of  temperature.  Hence  Volta  con- 
cluded that  slight  effects  due  to  a  wire  of  a  single  metal 
are  due  to  slight  difference  in  its  condition.  This  new  elec- 
tricity, declared  Volta,  might  as  well  be  called  "  metallic  "  as 
"animal."  The  strongest  proof  of  his  contact  theory  was 
given  by  means  of  his  condensing  electroscope.  This  was  a 
gold4eaf  electroscope  combined  with  a  small  condenser.  A 
feeble  source,  like  that  in  a  compound  bar  of  two  dissimilar 
metals,  could  supply  considerable  electricity  to  the  condenser 
without  materially  raising  its  potential.  But  when  the  upper 
plate  of  the  condenser  was  removed,  the  potential  rose  and 
the  leaves  diverged.  This  experiment  seemed  to  prove  that 
electricity  was  generated  at  the  contact  of  the  two  different 
metals,  one  metal  becoming  positively  charged,  the  other 
negatively. 

On  March  20,  1800,  Volta  wrote  a  letter  to  Joseph  Banks, 
then  president  of  the  Eoyal  Society  of  London,  in  which  he 
describes  the  voltaic  pile,  called  by  him  "organe  electrique 
artificiel"  in  distinction  to  the  "organe  electrique  natureP' 
of  the  torpedo.^  Two  dissimilar  plates,  say  zinc  and  copper, 
were  placed  in  contact.  Over  this  went  a  piece  of  flannel  or 
blotting  paper  moistened  with  water  or  brine.     Then  followed 

1  Fhil  Trans.,  1800,  p.  405. 


ELECTRICITY   AND    MAGNETISM  135 

auotlier  pair  of  zinc  and  copper  plates,  and  so  on,  each  pair  of 
plates  being  separated  by  a  moist  conductor.  Such  a  pile, 
consisting  of  a  dozen  or  more  pairs  of  plates,  multiplied  the 
effect  of  a  single  pair.  In  the  same  letter  Yolta  explains  the 
^^couronne  de  tasses"  or  "crown  of  cups."  It  consisted  of  cups 
containing  brine  or  dilute  acid,  into  which  dipped  strips  half 
zinc  and  half  copper.  The  zinc  end  of  one  strip  dipped  into  one 
cup,  the  copper  end  into  another.    This  is  the  first  voltaic  cell. 

Six  weeks  after  Yolta  had  written  that  memorable  letter, 
the  first  pile  was  constructed  in  England  by  William  Nicholson 
and  Sir  Antliony  Carlisle,  and  on  May  2,  the  decomposition  of 
water  by  it  was  observed.  This  experiment  was  the  founda- 
tion of  electro-chemistry.  It  was  described  in  Nicliolsorv' s 
Journal  for  July,  1800,  and  appeared  before  Volta's  own 
account  of  the  voltaic  pile  was  printed  in  the  Philosophical 
Transactions}  Volta's  researches  met  with  immediate  appre- 
ciation. As  early  as  1791  he  was  elected  member  of  the  Royal 
Society  of  London.  In  1801  Napoleon  called  him  to  Paris  to 
perform  before  the  Institute  his  experiments  on  the  pile.  The 
French  awarded  him  a  gold  medal. 

The  controversy  between  Volta  and  Galvani  divided  elec- 
tricians into  two  hostile  parties.  The  most  prominent  of 
G-alvani's  supporters  was  Alexander  von  Humboldt  in  Ger- 
many ;  the  most  prominent  of  Volta's  were  Coulomb  and  other 
French  physicists.  The  contact  theory  was  applied  to  the 
explanation  of  the  voltaic  cell.  This  theory  has  been  a  bone  of 
contention  from  that  time  to  the  present.  Only  in  very  recent 
years  has  it  finally  succumbed  to  a  modern  chemical  theory. 

1  The  electric  decomposition  of  water  was  accomplished  at  an  earlier 
date  by  Dr.  Ash  at  Oxford,  Fabbroni  in  Florence,  Cr^ve  in  Mainz,  but 
Nicholson  and  Carlisle  were  the  first  to  systematically  study  the  phe- 
nomenon and  to  prove  that  the  separated  gases  actually  were  hydrogen 
ind  oxygen.     See  Hoppe,  op.  cit.,  pp.  132-139. 


136  A   HISTOKY   OF   PHYSICS 

SOUND 

Joseph  Sauveur  (1653-1716)  carried  on  important  researches 
in  acoustics.  He  was  born  at  La  Fleclie.  At  the  age  of 
seventeen  he  travelled  on  foot  to  Paris  to  seek  his  fortune.  In 
1686  he  became  professor  of  mathematics  at  the  College  Eoyal. 
He  was  a  stammerer  and  had  such  a  poor  ear  for  music  that 
he  could  compare  pitches  only  with  the  assistance  of  musi- 
cians.-^ Yet  his  papers  on  acoustics,  published  in  the  Memoirs 
of  the  Academy,  1700-1703,  are  very  important.  Independ- 
ently of  Noble  and  Pigott,  he  discovered  overtones  in  strings. 
He  used  paper  riders  to  locate  nodes  and  anti-nodes.  He 
observed  sympathetic  vibrations  and  gave  a  correct  explana- 
tion of  beats.  He  tuned  two  organ  pipes  in  the  ratio  24  :  25, 
and  observed  four  beats  per  second.  Prom  this  he  concluded 
that  the  pipe  of  higher  pitch  made  100  vibrations  per  second. 
He  determined  rates  of  vibration  with  considerable  precision.'^ 
Vittorio  Francesco  Stancari  of  Bologna  made  such  measure- 
ments by  the  use  of  toothed  wheels.^ 

The  early  development  of  the  siren  took  place  in  England. 
The  experiments  of  Eobert  Hooke  were  continued  between 
1793  and  1801  by  John  Robinson,  professor  of  physics  at  the 
University  of  Edinburgh.  A  wheel  was  made  to  strike  in 
rapid  succession  the  teeth  of  a  pinion,  so  as  to  force  out  a 
portion  of  air  from  between  them;  or  a  pipe  through  which 
air  was  passing  was  alternately  opened  and  shut  by  the  revo- 
lution of  a  stopcock  or  valve.* 

1  RosENBERGER.  Part  II.,  p.  269. 

2  Consult  further  E.  Mach  on  Joseph  Sauveur  in  Mittheil.  d.  deutsch. 
math.  Ges.  zu  Prag,  1892  ;  abstracted  in  Poske's  ZeitscJir.,  Vol.  VI.,  pp. 
39-41. 

3  Ernst  Robel,  Die  Sirenen,  Ein  Beitrag  zur  Entwickelungsgeschichte 
der  Akustik,  Berlin,  1891,  Theil  I.,  p.  5. 

^Ibidem,  pp.  7-10.  Consult  also  Robinson,  "Temperament  of  the 
Scale  of  Music"  in  third  edition  of  the  Encyclopcedia  Britannica; 
Thomas  Young,  Lectures  on  Nat.  Phil.,  London,  1807,  Vol.  I.,  p.  378. 


THE   NINETEENTH   CENTURY 

In  physical  speculation  the  nineteenth,  century  has  over« 
thrown  the  leading  theories  of  the  previous  one  hundred 
years,  and  has  largely  built  anew  on '  the  older  foundations 
laid  during  the  seventeenth  century.  The  emission  theory  of 
light  gave  way  to  the  wave  theory ;  the  substance  called 
^'  caloric  "  was  set  aside,  and  the  fact  was  established  that  heat 
is  due  to  molecular  motion.  The  imponderables,  assumed  to 
exist  by  the  advocates  of  the  one-fluid  and  the  two-fluid 
theories  of  electricity,  were  discarded  in  favour  of  the  view 
that  the  phenomena  of  electricity  and  magnetism  are  to  be 
explained,  in  some  way,  by  the  existence  of  pulsations  and 
strains  in  the  luminiferous  ether.  The  efiluvia  of  a  magnet, 
capable  of  passing  through  glass  without  resistance,  are  of 
interest  now  only  to  the  historian.  The  chemical  substance 
phlogiston  is  no  more.  Of  the  half  dozen  imponderables 
which  filled  space  during  the  eighteenth  century,  only  one 
remains ;  and  this  one,  apparently,  has  proved  its  right  of 
existence.  By  it,  the  two  great  branches  of  light  and  electro- 
magnetism  are  becoming  practically  one.  Notwithstanding 
the  enormous  multiplication  of  observed  phenomena,  we  are 
simplifying  our  interpretation  of  them  by  bringing  into  con- 
sistent and  comprehensive  order  that  which  formerly  seemed 
to  be  capricious  and  isolated.  The  very  fact  that  intimate 
relations  are  perceived  to  exist  between  wide  realms  of  phys- 
ics, once  thought  to  be  perfectly  distinct,  seems  to  show  that 
we  are  moving  in  the  right  direction.  Eadiant  energy  has 
developed  into  a  subject  of  central  importance. 

Stimulated  and  aided  by  the  progress  of  its  sister  science 

137 


138  A   HISTORY    OF   PHYSICS 

chemistry,  physics  has  made  marvellous  progress  during  the 
past  one  hundred  years.  At  the  beginning  of  the  century  the 
chemist  with  his  balance  had  established  the  law  of  the  Conser- 
vation of  Mass.  Then  came  the  physicist  and  set  forth  in  bold 
relief  the  all-embracing  principle  of  the  Conservation  of  Energy. 

No  epoch  has  seen  such  a  vast  army  of  scientific  workers, 
or  beheld  the  acquisition  of  such  extensive  experimental 
knowledge  on  all  physical  subjects.  Theory  and  practice 
have  gone  hand  in  hand.  Steam  and  electricity  have  been 
made  to  minister  to  the  needs  and  comfort  of  mankind. 

Nevertheless  it  takes  no  fine  acumen  to  perceive  that  much 
remains  to  be  done.  It  is  left  for  the  twentieth  or  some  later 
century  to  reveal  fully  the  mysteries  of  the  structure  of  mat- 
ter and  the  ether.  Are  not  both  of  them  dynamical  systems, 
subject  to  the  laws  of  motion,  of  momentum,  and  energy  ? 
Present  indications  suggest  this  view. 

To  this  scientific  advance  all  leading  European  nations 
have  contributed.  In  Great  Britain  the  new  period  of  pro- 
ductiveness was  ushered  in  when  the  doting  attitude  of  Eng- 
lishmen toward  Newton  was  changed,  and  the  truth  was 
perceived  that  no  human  mind,  however  great,  can  be  infalli- 
ble on  all  points.  Among  the  earlier  scientists  of  this  time 
are  the  Herschels,  Thomas  Young,  Sir  Humphry  Davy,  and 
Sir  David  Brewster. 

Germany,  impoverished,  devastated,  and  politically  shattered 
by  religious  struggles,  began  after  the  Napoleonic  wars  to 
recover  and  to  put  forth  extraordinary  scientific  effort.  The 
attitude  of  German  physicists  of  the  early  part  of  this  century 
toward  philosophers  and  mathematicians  was  grotesque.  The 
obscure  and  undemonstrated  assertions  of  the  philosophers 
Hegel  and  Schelling  worked  injuriously  upon  science.-^     But 

1  Helmholtz  in  Wiedemann''s  Annalen,  Neue  Tolge,  Vol.  54,  1895, 
pp.  2  et  seq.     Helmholtz  says  in  his  lecture,  "On  the  relation  of  natural 


THE  NINETEENTH  CENTURY  139 

a  reaction  set  in.  There  arose  in  Berlin  an  empirical  schooV 
of  scientists,  comprising  Poggendorff,  Riess,  Dove,  H.  G.  Mag- 
nus. They  looked  with  contempt  upon  metaphysical  obscu- 
rantism. Magnus,  the  leader  of  this  school,  did  much  toward 
the  evolution  of  the  modern  physical  laboratory. 

Strange  to  say,  Magnus  would  have  nothing  to  do  with 
mathematics.  This  one-sided  and  ill-founded  conception  of 
the  use  of  mathematics  in  physical  research  was  not  shared 
by  his  great  pupils  Kronig,  Clausius,  and  Helmholtz.  Nor 
did  all  German  contemporaries  of  Magnus  shun  mathe- 
matical physics.  This  branch  was  cultivated  at  Gottingen 
by  Gauss  and  Wilhelm  Weber,  and  at  Konigsberg  by 
Ernst  Eranz  Neumann.  The  first  movement  toward  unity 
of  action  among  physical  experimentalists  and  mathemati- 
cians took  place  in  the  organization  at  Berlin  in  1844  of 
the  Physical  Society,  which  grew  out  of  a  physical  "collo- 
quium "  held  by  Magnus.^ 

France,  at  the  beginning  of  the  nineteenth  century,  possessed 
an  array  of  scientific  men  of  unsurpassed  brilliancy.  We  need 
mention  only  Lagrange,  Laplace,  Fresnel,  Arago,  Biot,  Carnot, 
Fourier.  Not  till  the  middle  of  this  century  did  some  of  the 
other  countries  equal  her  in  scientific  productiveness. 

In  the  United  States  comparatively  little  was  achieved 
before  the  last  quarter  of  the  century,  and  that  little  failed, 
at  the  time,  to  catch  the  eye  of  the  scientific  public  abroad. 

science  to  general  science,"  Popular  Lectures^  transl.  by  E.  Atkinson, 
London,  1873,  p.  7  :  "Hegel  .  .  .  launched  out,  with  unusual  vehemence 
and  acrimony,  against  the  natural  philosophers  and  especially  against  Sir 
Isaac  Newton,  as  the  first  and  greatest  representative  of  physical  investi- 
gation. The  philosophers  accused  tlie  scientific  men  of  narrowness ;  the 
scientific  men  retorted  that  the  philosophers  were  crazy."  Consult  also 
Rudolph  Yirchow,  "Transition  from  the  Philosophic  to  the  Scientific 
Age,"  Smithsonian  Report^  1894,  pp.  681-695. 

1  G.  Wiedemann  in  Wied.  Annal.,  Vol.  39,  1890,  "  Vorwort." 


140  A  HISTORY   OF   PHYSICS 

LIGHT 

We  are  indebted  to  Thomas  Young  (1773-1829),  a  native  of 
Milverton,  Somersetshire,  for  the  revival  of  the  undulatory 
theory  of  light  after  a  century  of  neglect.  This  great  scientist 
had  an  extraordinary  childhood.  He  could  read  with  con- 
siderable fluency  at  the  age  of  two.  When  four  years  old  he 
had  read  the  Bible  twice  through;  when  six  he  could  repeat 
the  whole  of  Goldsmith's  Deserted  Village.  He  devoured 
books,  whether  classical,  literary,  or  scientific,  in  rapid  succes- 
sion ;  and,  strange  to  say,  he  grew  up  with  unimpaired  physical 
and  intellectual  powers.  At  about  sixteen  he  abstained  from 
the  use  of  sugar  on  account  of  his  opposition  to  the  slave-trade. 
At  nineteen  he  entered  upon  a  medical  education,  which  was 
pursued  first  in  London,  then  in  Edinburgh,  Gottingen,  and 
finally  at  Cambridge.  In  1800  he  began  medical  practice  in 
London.  The  year  following  he  accepted  the  office  of  Professor 
of  Natural  Philosophy  in  the  Royal  Institution,  the  metro- 
politan school  of  science  established  in  the  year  preceding 
by  Count  E-umford.  He  held  this  position  two  years.  From 
January  to  May,  1802,  he  delivered  there  a  series  of  lectures. 
These  and  a  later  series  were  published  in  1807  under  the  title, 
Lectures  on  Natural  Philosophy  and  the  Mechanical  Arts,  a 
treatise  still  worthy  of  perusal.  In  1802  he  was  appointed 
Foreign  Secretary  of  the  Ro5^al  Society.  This  office  he  held 
for  the  remainder  of  his  life. 

Young's  earliest  researches  were  on  the  anatomical  and  opti- 
cal properties  of  the  eye.  Then  followed  the  first  epoch  of 
optical  discovery,  1801-1804.  His  theory  was  laughed  at,  and 
he  proceeded  to  other  studies.  The  twelve  succeeding  years 
were  given  to  medical  practice  and  to  the  study  of  philology, 
especially  the  decipherment  of  hieroglyphic  writing.  But 
when  Fresnel,  in  France,  began  to  experiment  on  light  and  to 


LIGHT  141 

bring  into  prominence  tlie  theory  of  Yonng,  then  the  lattei 
resumed  his  early  studies,  and  entered  upon  his  second  epoch 
of  optical  investigation. 

In  1801  Young  read  before  the  Eoyal  Society  a  paper  on  the 
colour  of  thin  plates,  in  Avhich  he  expressed  himself  strongly  in 
favour  of  the  undulatory  theory  of  light.  The  great  step  taken 
in  this  article  is  the  introduction  of  the  principle  of  interfer- 
ence. "When  two  undulations,  from  different  origins,  coin- 
cide either  perfectly  or  very  nearly  in  direction,  their  joint 
effect  is  a  combination  of  the  motions  belonging  to  each."  ^ 
Imperfect  hints  of  this  principle  occur  in  Robert  Hooke's 
Micrograpliia,  but  Young  was  unaware  of  these  until  after  he 
had  arrived  at  the  notion  independently.  Young  was  the  first  to 
make  a  thorough  application  of  it  to  sound  and  light.  By  this 
principle  he  explained  the  colours  of  thin  plates  and  the  dif- 
fraction colours  of  scratched  or  "  striated  surfaces."  ^  Young's 
observations  were  made  with  great  exactness,  but  the  mode  of 
exposition  in  these,  as  in  most  of  his  memoirs,  was  condensed 
and  somewhat  obscure.  His  papers,  containing  the  great  prin- 
ciple of  interference,  constituted  by  far  the  most  important 
publication  on  physical  optics  issued  since  the  time  of  ISTewton. 
Yet  they  made  no  impression  upon  the  scientific  public.  They 
were  attacked  violently  by  Lord  Brougham  in  ISTos.  II.  and 
IX.  of  the  Edinburgh  Review.  Young's  articles  were  declared 
to  contain  "nothing  which  deserves  the  name  either  of  ex- 
periment or  discovery,"  to  be  "destitute  of  every  species  of 
merit."     "  We  wish  to  raise  our  feeble  voice,"  says  Brougham, 

1  Miscellaneous  Works  of  the  Late  Thomas  Young,  edited  by  George 
Peacock,  London,  1855,  Vol.  L,  p.  157.     See  also  p.  170. 

2  The  colours  of  scratches  on  polished  surfaces  were  observed  first  by 
Robert  Boyle.  Later,  examples  of  lines  drawn  on  glass  were  pro- 
duced by  Mr.  Barton,  which,  when  transferred  to  steel  —  as  in  the  case 
of  the  buttons  which  are  known  by  his  name  —  produce  a  very  brilliant 
effect  of  coloration.     George  Peacock's  Life  of  Dr.  Young,  1855,  p.  149 


142  A  HISTORY   OF   PHYSICS 

^''against  innovations  that  can  have  no  other  effect  than  to 
check  the  progress  of  science."  After  exposing  the  law  of 
interference  as  "absurd"  and  "illogical,"  the  reviewer  says, 
"We  now  dismiss,  for  the  present,  the  feeble  lucubrations  of 
this  author,  in  which  we  have  searched  without  success  for 
some  traces  of  learning,  acuteness,  and  ingenuity,  that  might 
compensate  his  evident  deficiency  in  the  powers  of  solid  think- 
ing, calm  and  patient  investigation,  and  successful  develop- 
ment of  the  laws  of  nature,  by  steady  and  modest  observation 
of  her  operations."^  Young  issued  an  able  reply,  published 
in  the  form  of  a  pamphlet,  which  failed  to  turn  public  opinion 
in  favour  of  his  theory,  because,  as  he  said,  "one  copy  only 
was  sold."^  Says  TyndalV  "For  twenty  years  this  man  of 
genius  was  quenched  —  hidden  from  the  appreciative  intellect 
of  his  countrymen  —  deemed  in  fact  a  dreamer,  through  the 
vigorous  sarcasm  of  a  writer  who  had  then  possession  of  the 
public  ear.  .  .  .  To  the  celebrated  Frenchmen,  Fresnel  and 
Arago,  he  was  first  indebted  for  the  restitution  of  his  rights." 
Augustin  Jean  Fresnel  (1788-1827)  was  born  at  Broglie  in 
Normandy.  He  advanced  very  slowly  in  his  studies,  being  at 
eight  years  of  age  scarcely  able  to  read.'*  The  state  of  his 
health  was  always  delicate.  Unlike  Thomas  Young,  he  gave 
no  promise  of  becoming  a  great  savant.  At  the  age  of  thirteen 
he  went  to  the  central  school  at  Caen,  at  sixteen  to  the  Poly- 
technic  School  in  Paris,  then  to  the  Ecole  des  ponts  et  chaiissees. 
Then  he  served  as  government  engineer  for  about  eight  years. 
He  was  a  strong  Eoyalist,  and  joined  the  army  organized  to 
oppose  the  return  of  Napoleon  from  Elba.     As  a  result  he  was 

1  Edinhurgh  Beview,  No.  IX.,  6tli  ed.,  Vol.  V.,  p.  103  ;  Toung^s  Works^ 
I.,  p.  193. 

2  Ibidem,  I.,  215. 

3  Six  Lectures  on  Light,  2(i  ed.,  New  York,  1877,  p.  51. 

4  F.  Arago,  Biographies,  2d  Series,  Boston,  1859,  p.  176. 


LIGHT  143 

deprived  of  his  office.  On  the  reinstatement  of  Louis  XYIII. 
Fresnel  obtained  a  new  position  as  engineer.  He  entered  upon 
his  experimental  researches  in  1815.  A  letter  of  December, 
1814,  contains  the  following :  "  I  do  not  know  what  is  meant 
by  polarization  of  light."  Within  a  year  he  transmitted  to 
the  Academy  an  important  memoir  on  diffraction  (October, 
1815).  Other  memoirs  followed  in  rapid  succession.^  By 
placing  a  wire  in  a  beam  of  light  diverging  from  a  point,  the 
distances  of  the  resulting  fringes  from  the  axis  of  the  beam 
were  accurately  measured.  He  noticed,  as  Young  had  done 
earlier,  the  disappearance  of  the  bands  within  the  shadow, 
when  the  light  which  passed  on  one  side  of  the  wire  was  cut 
off  before  it  reached  the  screen.  Fresnel  was  led  to  the  dis- 
covery of  the  principle  of  interference,  without  being  aware 
that  Young  had  achieved  this  more  than  thirteen  years  before. 
Many  physicists  were  not  inclined  to  admit  that  the  phenom- 
ena were  due  to  interference.  Diffraction  fringes  had  been 
known  since  the  time  of  Grimaldi,  and  had  been  explained  on 
the  emission-theory  by  means  of  hypothetical  laws  of  attrac- 
tion and  repulsion  between  the  light  corpuscles  and  the  edges 
of  the  object  causing  diffraction.  To  remove  these  objections 
Fresnel  designed  the  memorable  experiment  which  yielded  two 
small  sources  of  light,  without  resorting  to  apertures  or  edges 
of  opaque  obstacles.  By  the  use  of  two  plane  metallic  mir- 
rors, forming  with  each  other  an  angle  of  nearly  180°,  he 
avoided  diffraction,  and  yet  with  the  reflected  beams  produced 
interference. 

Arago  and  Poinsot  were  commissioned  to  report  on  FresnePs 
first  memoir.  Arago  entered  upon  the  subject  with  zeal  and 
became  the  first  convert  in  France  to  the  undulatory  theory. 


1  Consult  (Euvres  completes  d^Augustin  Fresnel,  Paris,  1866,  in  three 
volumes,  with  introduction  by  Emile  Yerdet. 


144  A   HISTORY   OF   PHYSICS 

Some  of  Presnel's  mathematical  assumptions  were  not  satis- 
factory ;  hence  Laplace,  Poisson,  and  others  belonging  to  the 
strictly  mathematical  school  at  first  disdained  to  consider  the 
theory.  By  their  opposition  Fresnel  was  spurred  to  greater 
exertion.  Young  had  not  verified  his  explanations  by  ex- 
tensive numerical  calculations.  Fresnel  applied  mathemati- 
cal analysis  to  a  much  greater  extent,  and  the  undulatory 
theory  began  to  carry  conviction  to  many  minds.  He  gave  a 
complete  answer  to  the  old  objection  against  the  wave  theory, 
that  the  latter  could  not  explain  the  existence  of  shadows  or 
the  approximate  rectilinear  propagation  of  light. 

Unlike  Young,  Fresnel  made  extensive  use  of  Huygens's 
principle  of  secondary  waves,  stated  by  Fresnel  as  follows : 
"  The  vibrations  of  a  luminous  wave  at  any  one  of  its  points 
may  be  considered  as  the  sum  of  the  elementary  movements 
conveyed  to  it  at  the  same  moment,  from  the  separate  action 
of  all  the  portions  of  the  unobstructed  wave  considered  in  any 
one  of  its  anterior  positions."  ^ 

It  was  Arago  who  first  drew  Fresnel's  attention  to  Young's 
researches,  and  who  sent  to  the  English  physician  the  first 
memoir  of  the  French  savant.  It  is  a  pleasure  to  note  the 
absence  of  bitter  contests  of  priority.  Fresnel  writes  Young 
in  1816  :  "  But  if  anything  could  console  me  for  not  having 
the  advantage  of  priority,  it  was  for  me  to  have  met  a  savant 
who  has  enriched  physics  with  so  great  a  number  of  impor- 
tant discoveries,  and  has  at  the  same  time  contributed  greatly 
to  strengthen  my  confidence  in  the  theory  that  I  have 
adopted."  ^  Young  writes  to  Fresnel,  October  16, 1819  :  "  I  re- 
turn a  thousand  thanks,  Monsieur,  for  the  gift  of  your  admi- 
rable memoir,  which  surely  merits  a  very  high  rank  amongst 


1  G.  Peacock,  Life  of  Thomas  Young,  London,  1855,  p.  167. 

2  Young's  Works,  Vol.  I.,  p.  378. 


i 


LIGHT  145 

fche  papers  which  have  contributed  most  to  the  progress  of 

optics.''  ^ 

Let  Tis  proceed  to  double  refraction  and  the  polarization 
of  light.  Double  refraction  had  been  observed  in  Iceland 
spar  by  Erasmus  Bartholinus.  Polarization  had  been  studied 
by  Huygens  and  jSTewton.  Huygens  had  stated  the  true  law 
of  extraordinary  refraction  in  uniaxal  crystals.  The  property 
of  "  two-sidedness  "  or  "  polarization  "  was  known  to  them  as 
an  isolated  fact  observed  only  in  connection  with  double 
refraction.  A  century  elapsed  and  then  Mains  observed  that 
polarization  may  accompany  reflection.  Thus  light  may  be 
polarized  in  other  ways  than  by  the  action  of  crystallized 
bodies. 

Etienne  Louis  Malus  (1775-1812)  was  born  in  Paris.  He  was 
educated  as  a  military  engineer  and  served  in  the  French  army 
in  Germany  and  Egypt.  Later,  during  his  superintendence  of 
the  work  then  in  progress  at  Antwerp  and  at  Strassbnrg,  he 
found  time  to  undertake  the  investigation  of  a  prize  question 
proposed  by  the  French  Institute,  calling  for  a  mathematical 
theory  of  double  refraction.  By  accident  he  was  led  to  the 
discovery  alluded  to  above.  He  looked  through  a  piece  of 
crystal  at  the  image  of  the  sun  reflected  from  the  windows  of 
the  Luxembourg  Palace,  to  the  house  in  the  Eue  d'Enfer, 
where  he  lived,  and  was  much  surprised  to  find  one  of  the 
double  images  disappear  for  a  certain  position  of  the  crystal.^ 
He  tried  to  explain  the  singular  phenomenon  by  some  modifi- 
cation of  the  light  undergone  in  traversing  the  atmosphere. 
But  when  night  came,  he  found  that  the  light  of  a  taper,  fall- 
ing upon  the  surface  of  water  at  an  angle  of  36°,  acted  simi- 
larly and,  in  fact,  was  polarized.  Moreover,  if  the  two  rays 
from  calc-spar  fell  simultaneously  on  the  surface  of  water  at 

1  Ibidem,  Vol.  I.,  p.  393.  2  Young's  Works,  Vol.  II.,  593. 


146  A   HISTORY  OF   PHYSICS 

an  angle  of  36°,  and  if  the  ordinary  ray  was  partly  reflected, 
then  the  extraordinary  ray  was  not  reflected  at  all,  and  vice 
versa.  Thus,  in  one  evening.  Mains  created  a  new  branch  of 
modern  physics. 

At  this  time  no  explanation  of  polarization  had  been  given 
by  the  wave  theory,  which  was  in  great  danger  of  being  over- 
thrown by  the  new  mass  of  evidence  furnished  by  Mains. 
Thomas  Young  wrote  in  1811  to  Malus  (who  was  a  pronounced 
partisan  of  the  emission  theory) :  "  Your  experiments  demon- 
strate the  insufficiency  of  a  theory  (that  of  interferences),  which 
I  had  adopted,  but  they  do  not  prove  its  falsity. ^^^  As  Whewell 
says,^  this  was  without  doubt  "  the  darkest  time  of  the  history 
of  the  theory."  Young  did  not  conceal  the  difficulty ;  nor  did 
he  despair  of  reconciling  a  seeming  contradiction.  Six  years 
passed,  then  light  began  to  dawn.  On  January  12, 1817,  Young 
wrote  to  Arago,  "  It  is  a  principle  in  this  theory,  that  all  undu- 
lations are  simply  propagated  through  homogeneous  mediums 
in  concentric  spherical  surfaces  like  the  undulations  of  sound, 
consisting  simply  in  the  direct  and  retrograde  motions  of  the 
particles  in  the  direction  of  the  radius,  with  their  concomitant 
condensations  and  rarefactions.  And  yet  it  is  possible  to 
explain  in  this  theory  a  transverse  vibration,  propagated  also 
in  the  direction  of  the  radius,  and  with  equal  velocity,  the 
motions  of  the  particles  being  in  a  certain  constant  direction 
with  respect  to  that  radius  ;  and  this  is  a  polarization.^^  ^  This 
was  a  happy  suggestion  which  made  it  possible  to  see  how  a 
ray  could  exhibit  two-sidedness.  Later,  instead  of  the  ''  con- 
stant direction  "  spoken  of  by  Young,  the  particular  direction 
transverse  to  the  ray  was  fixed  upon.  Fresnel  arrived  at  this 
mode  of   explanation  independently,  but  its  publication  ap- 

1  Arago' s  Biographies,  2d  Series,  1859,  p.  159. 

2  Inductive  Sciences^  New  York,  1858,  Vol.  II. ,  p.  100. 
8  Young's  Works,  Vol.  I.,  p.  383. 


LIGHT  147 

peared  after  Young's.  Some  idea  of  the  difficulty  encountered 
in  grasping  the  notion  of  transverse  vibrations  is  obtained 
from  Arago's  narration  to  Whewell,  "  that  when  he  [Arago] 
and  Fresnel  had  obtained  their  joint  experimental  results  of 
the  non-interference  of  oppositely  polarized  pencils,  and  when 
Fresnel  pointed  out  that  transverse  vibrations  were  the  only 
possible  translation  of  this  fact  into  the  undulatory  theory,  he 
himself  protested  that  he  had  not  courage  to  publish  such  a 
conception;  and,  accordingly,  the  second  part  of  the  Memoir 
was  published  in  Fresnel's  name  alone."  ^  Fresnel  advanced 
the  whole  subject  of  polarized  light.  The  rich  colours  produced 
by  polarized  light  passing  through  certain  crystals  were  dis- 
covered by  Arago  in  1811.  Partisans  of  the  two  rival  optical 
theories  hastened  to  find  explanations  of  this  phenomenon  of 
depolarization.  On  the  undulatory  theory  explanations  were 
given  first  by  Young,  then  more  fully  by  Arago  and  Fresnel. 
On  the  corpuscular  theory,  the  facts  were  accounted  for  by 
Biot  in  a  complicated  research  of  great  mathematical  elegance. 
This  was  received  favourably  by  Laplace  and  other  mathema- 
ticians, who  found  the  speculations  of  Biot  more  congenial  to 
their  habits  of  thought  than  those  of  Fresnel.  Arago  entered 
the  lists  against  Biot,  and  the  discussion  was  carried  on  with 
such  bitterness  that  the  two  physicists,  once  intimately  asso- 
ciated, became  wholly  estranged.^  About  1816  Biot  discovered 
that  plates  of  tourmaline  show  double  refraction,  but  absorb 
the  ordinary  ray.  This  led  him  to  the  construction  of  the 
well-known  tourmaline  tongs  for  the  study  of  polarization 
phenomena.  He  gave  also  the  important  laws  of  rotary 
polarization  and  their  application  to  the  analysis  of  various 
substances. 

^  Inductive  Sciences^  Yol.  II.,  p.  101. 

2  Proceedings  of  the  American  Academy  of  Arts  and  Sciences,  Vol.  VI., 
1862-1865,  p.  16  et  seq.,  "Jean  Baptiste  Biot." 


148  A   HISTORY   OF   PHYSICS 

The  phenomena  of  polarized  light  in  crystals  were  ex 
amined  with  great  success  by  Sir  David  Brewster  (1781-1868). 
Although  educated  for  the  Church,  he  never  engaged  in  its 
active  duties.  In  1799  he  was  induced  by  his  fellow-student, 
Brougham,  to  repeat  and  study  Newton's  experiments  on 
diffraction.  Erom  that  time  on  Brewster  was  engaged  almost 
continually  in  original  research.  He  became  professor  of 
physics  at  St.  Andrews,  and  later,  principal  of  the  University 
of  Edinburgh.  In  1819  he  established,  in  connection  with 
Jameson,  the  Edinburgh  Philosophical  Journal.  He  was  the 
leading  organizer  of  the  British  Association  for  the  Advance- 
ment of  Science,  which  held  its  first  meeting  at  York  in  1831. 
He  became  famous  as  the  inventor  of  the  kaleidoscope,  for 
which  the  demand  in  both  England  and  America  was  greater, 
for  a  time,  than  could  be  met.  Brewster,  like  Biot,  was  never 
friendly  to  the  undulatory  theory.  "The  discoverer  of  the 
law  of  polarization  of  biaxal  crystals,  of  optical  mineralogy, 
and  of  double  refraction  by  compression "  was  in  a  frame  of 
mind  to  assert,  even  after  the  maturer  researches  of  Young, 
Fresnel,  and  Arago  had  been  given  to  the  world,  that  "  his 
chief  objection  to  the  undulatory  theory  of  light  was  that  he 
could  not  think  the  Creator  guilty  of  so  clumsy  a  contrivance 
as  the  filling  of  space  with  ether  in  order  to  produce  light."  ^ 

After  1825  the  emission  theory,  though  still  supported  by 
several  scientists  of  prominence,  was  abandoned  by  the  majority 
of  physicists,  especially  by  the  younger  men.  Nevertheless, 
the  crucial  test,  which  destroyed  once  for  all  the  validity  of  the 
emission  theory,  was  not  performed  until  the  middle  of  the 
century.  According  to  the  emission  theory, the  velocity  of 
light  is  greater  in  an  optically  denser  medium,  while,  accord- 
ing to  the  undulatory  theory,  it  is  smaller.     Wheatstone,  who 

1  Tyndall,  8ix  Lectures  on  Light,  2d  ed.,  New  York,  1877,  p.  49. 


LIGHT  149 

as  early  as  1834  had  been  determining  the  duration  of  the 
electric  spark  by  aid  of  rotating  mirrors,  suggested  that  the 
same  method  might  be  used  to  ascertain  the  velocity  of  light 
and  to  find  out  whether  the  speed  was  greater  in  the  more 
refracting  medium.  The  idea  was  taken  up  by  Arago,  but  as 
his  eyesight  was  poor,  the  undertaking  was  left  to  younger 
men.  The  mechanical  difficulties  were  great ;  a  mirror  must 
be  made  to  rotate  at  a  speed  of  over  one  thousand  revolu- 
tions per  second.  By  some,  Arago's  project  was  considered 
chimerical,  because  it  was  thought  impossible  for  the  eye  to 
seize  the  instantaneous  image  of  a  flash  reflected  from  a  mirror 
rotating  with  such  enormous  speed.  Bertrand  remarked  that 
"an  attentive  and  assiduous  observer  may,  according  to  com- 
putations of  M.  Babinet,  hope  to  catch  the  ray  once  in  three 
years."  ^  The  experiment  was  undertaken  by  Foucault.  He 
adopted  the  combination  of  apparatus  now  described  in  almost 
every  general  treatise  on  physics,  by  which  the  difficulty 
mentioned  above  was  removed.^  The  success  of  his  experi- 
ments was  announced  to  the  Academy  of  Sciences,  May  6, 
1850.  He  found  the  velocity  of  light  in  water  to  be  less  than 
in  air ;  from  that  moment  Newton's  emission  theory  was  dead. 
Jean  Leon  Foucault  (1819-1868)  was  born  in  Paris.  He 
studied  medicine,  but  between  the  years  1845  and  1849  entered 
upon  physical  researches.  At  this  time  he  worked  in  conjunc- 
tion with  Fizeau.  After  their  separation,  each  made  deter- 
minations of  the  velocity  of  light.  Foucault's  research  on  the 
velocity  in  air  relative  to  that  in  water,  mentioned  above,  was 
carried  on  at  his  pavilion  in  the  Rue  d'Assas,  and  was  sub- 
mitted by  him  in  1853  as  a  thesis  for  the  degree  of  Doctor  of 

1  Ph.    Gilbert,    Leon    Foucault^  sa    vie   et   son  oeuvre   scientifique, 
Bruxelles,  1879,  p.  32. 

2  For  details,   see  Delaunat,   "Essay  on  the  Velocity  of   Light,'' 
Smithsonian  Beport,  1864,  pp.  135-165. 


150  A   HISTORY   OF   PHYSICS 

Science.^  In  1851  Foucault  presented  a  memoir  giving  his 
famous  demonstration  of  the  rotation  of  the  earth  by  means  of 
the  pendulum.^  The  following  year  he  invented  that  marvel- 
lous piece  of  mechanism,  the  gyroscope.  In  1854  Napoleon  III. 
secured  a  place  for  him  at  the  Paris  Observatory  as  physicist. 
Much  was  contributed  by  Foucault  toward  greater  perfection 
of  astronomical  instruments.^ 

Foucault's  early  co-worker,  Hippolyte  Louis  Fizeau  (1819- 


1  Ph.  Gilbert,  oj9.  cit..,  p.  32. 

2  The  experiment  was  made  in  foar  places.  The  first  one  was  a  cellar 
two  metres  deep  at  his  pavilion  in  the  Rue  d'Assas.  A  brass  ball  weigh- 
ing five  kilogrammes  was  suspended  by  a  steel  wire.  The  ball  was  drawn 
aside,  held  in  that  position  by  a  thread  until  it  was  at  complete  rest,  then 
set  free  by  burning  the  thread.  The  pendulum  began  oscillating  in  a 
Jixed  vertical  plane,  making  thereby  the  fact  of  the  earth's  rotation 
experimentally  evident.  To  the  eye  the  plane  of  oscillation  seemed  to 
rotate  and  the  earth  to  be  at  rest.  Theory  indicated  that  the  angle  of 
this  apparent  motion  in  a  given  time  was  equal  to  the  angle  through 
which  the  earth  rotated  in  the  same  time,  multiplied  by  the  sine  of  the 
angle  of  latitude  of  the  place  where  the  experiment  was  made.  An 
accurate  verification  of  this  law  required  more  favourable  conditions. 
Arago  offered  Foucault  the  use  of  the  observatory  building,  where  a 
pendulum  eleven  metres  long  enabled  him  to  demonstrate  the  law  with 
exactitude.  Through  the  favour  of  Napoleon  III.,  the  Pantheon  was 
chosen  for  the  third  test,  A  ball  of  twenty-eight  kilogrammes  was  sus- 
pended there  by  a  wire  sixty-seven  metres  long  and  1.4  millimetres  thick. 
The  Pantheon  was  thronged  with  visitors.  The  fourth  exhibition  was 
made  at  the  Universal  Exposition  of  1855.  These  pendulum  experiments 
became  very  famous.  The  only  previous  record  of  similar  observations 
dates  from  the  time  of  the  Accademia  del  Cimento.  Viviani  is  credited 
with  the  statement,  "We  have  observed  that  all  pendulums  suspended 
by  a  single  thread  deviate  from  their  primitive  vertical  plane  and  do  so 
always  in  the  same  direction."  See  Ph.  Gilbert,  op.  cit.,  p.  55.  But 
there  is  nothing  to  show  that  the  Italian  had  divined  the  cause. 

3  Foucault  possessed  a  poorly  developed  body.  Says  Lissajous  :  "It 
seemed  as  if  nature  had  undertaken  to  establish  a  striking  contrast 
between  Foucault's  physique  and  his  intellectual  powers.  Who  could 
have  divined  the  man  of  genius  under  this  frail  appearance  ?"    Ibidem,  13. 


LIGHT  151 

1896)^  was  born  in  Paris.  Being  in  possession  of  a  fortune 
which  left  him  free  to  follow  his  own  inclinations,  he  devoted 
himself  to  physics.  The  means  for  his  researches  were  largely 
supplied  from  his  own  private  resources.  In  1849  he  made 
the  earliest  experimental  determination  of  the  absolute  velocity 
of  light.  E/omer's  and  Bradley's  measurements  had  been  based 
on  astronomical  observation.  Fizeau  rotated  a  toothed  wheel, 
which  intercepted  light  at  regular  intervals.  The  intermittent 
flashes  were  reflected  from  a  distant  fixed  mirror.  The 
research  was  carried  on  in  the  suburbs  of  Paris,  between 
Suresnes  and  Montmartre,  a  distance  of  8633  metres.^  His 
article  in  the  Comptes  Rendus  (Vol.  29,  p.  90)  appeared  in 
1849,  the  year  before  Foucault's  paper  on  the  relative  velocity 
of  light  in  air  and  water  (Vol.  30,  p.  551).  In  the  year  1862 
Poucault  applied  his  method  to  the  determination  of  the  abso- 
lute velocity,  and  found  values  surpassing  in  accuracy  all  pre- 
vious measurements.^ 

Pizeau  made  interesting  experiments  on  the  relative  motion 
of  ether  and  matter,  showing  that  the  ether  within  a  trans- 
parent medium  is  carried  forward  by  the  moving  medium,  but 
with  a  velocity  less  than  that  of  the  medium.  These  results 
have  been  confirmed  by  Michelson  and  Morley.'* 

Pizeau's  method  of  finding  the  velocity  of  light  was  adopted 
with  some  modifications  by  Alfred  Cornu  in  Paris  and  by 
James  Young  and  George  Forbes  in  England.  In  Cornu's 
experiments  of  1874  the  fixed  mirror  was  at  a  distance  of 
23  kilometres.^     Young  and  Porbes's  measurements,  published 


1  Nature^  Vol.  54,  1896,  p.  523 ;  P.  Larousse,   Grand  Dictionnaire 
Universel. 

2  Ph.  Gilbert,  op.  cit.,  p.  36. 

»  Comptes  Beudiis,  Vol.  55,  1862,  pp.  501,  792. 
.  *  Am.  Jour,  of  Sci.  (3),  Vol.  31,  p.  377,  1886. 
®  Annales  de  VObservatoire  de  Paris  {Memoires^  Vol.  13,  1876). 


152  A   HISTORY    OF   PHYSICS 

in  1882/  seemed  to  sliow  that  the  blue  rays  travel  about  1.8 
per  cent  faster  than  the  red.  The  correctness  of  this  result 
has  been  doubted.  If  true,  stars  should  appear  coloured  just 
before  and  after  an  eclipse ;  moreover,  Michelson,  by  Fou- 
cault's  method,  should  have  seen  a  spectral  drawing  out  of  the 
image  of  the  slit,  yielding  a  coloured  image  ten  millimetres  in 
width.^ 

The  best  determinations  of  light-velocity  have  been  made  in 
the  United  States.  In  1867  Simon  Newcomh  (born  1835),  then 
of  the  Naval  Observatory,  recommended  the  repetition  of 
Foucault's  experiment  that  closer  values  for  the  solar  parallax 
might  be  obtained.  A  preliminary  test  was  made  in  1878  by 
Albert  A.  Miclielson  (born  1852)  at  the  laboratory  of  the  Naval 
Academy  at  Annapolis.^  A  gift  of  $2000  enabled  him  to  con- 
tinue experimentation.  Measurements  were  taken  in  1879. 
At  Newcomb's  request  Michelson,  in  1882,  made  a  determina- 
tion at  the  Case  Institute  in  Cleveland,  Ohio.  The  main  diffi- 
culty in  Foucault's  experiments  had  been  that  the  deflection 
was  too  small  to  be  measured  accurately.  His  distance 
between  the  fixed  and  the  rotating  mirror  was  only  4  metres 
(though,  by  using  five  fixed  mirrors,  this  was  virtually  in- 
creased to  20  metres),  and  the  displacement  of  the  return 
image  was  only  .7  millimetre.  In  Michelson's  improved 
arrangement  the  return  image  was  displaced  through  133  milli- 
metres, or  nearly  200  times  that  obtained  by  Foucault. 

In  March,  1879,  Congress  voted  an  appropriation  of  f  5000 
for  experiments  to  be  made  under  the  direction  of  Simon  New- 


1  PMlos.  Trans.,  Part  I.,  1882. 

2  A.  A.  Michelson,  Astr.  Papers  for  the  Am.  Ephem.  and  Naut. 
Almanac,  Vol.  II.,  Part  IV.,  p.  237,  1885. 

3  Joseph  Lovering,  "  Address  on  Presentation  of  Rumford  Medal  to 
Prof.  A.  A.  Miclielson,"  in  Am.  Acad,  of  Arts  and  Science,  New  Series, 
Vol,  16,  1888-89,  p-  38-1.     We  h;»ve  taken  sAvcral  details  from  this  source. 


LIGHT  153 

comb.  The  movable  mirror  was  mounted  at  Fort  Meyer.  The 
fixed  mirror  was  placed  at  one  time  at  the  Naval  Observatory 
(distance,  2550.95  metres),  and  at  another  time  at  Washington 
Monument  (distance,  3721.21  metres).  Michelson  assisted  in 
the  operations  until  he  removed  to  Cleveland  in  the  autumn  of 
1880.  Observations  began  in  the  summer  of  1880,  and  were 
continued  until  the  autumn  of  1882,  the  most  favourable  days 
in  spring,  summer,  and  autumn  being  selected.  Only  during 
the  hour  after  sunrise  or  the  hour  before  sunset  were  the 
atmospheric  conditions  such  that  a  steady  image  of  the  slit 
could  be  obtained.  Altogether  504  sets  of  measurements  were 
made ;  276  by  ISTewcomb,  140  by  Michelson,  88  by  Holcombe.^ 

The  results  in  kilometres  per  second  obtained  for  the 
velocity  of  light  in  vacuo  are  as  follows :  Eizeau,  in  1849, 
315,000 ;  Foucault,  in  1862,  298,000 ;  Cornu,  in  1874,  298,500 ; 
Cornu,  in  1878,  300,400;  Young  and  Forbes,  in  1880-1881, 
301,382;  Michelson,  in  1879,  299,910;  Michelson,  in  1882, 
299,853 ;  Newcomb,  in  1882,  299,860,  when  using  only  results 
supposed  free  from  constant  error,  and  299,810  when  including 
all  observations.^ 

The  earliest  observation  of  dark  lines  in  the  solar  spectrum 
was  made  by  William  Hyde  Wollaston  (1766-1828),  a  London 
physician.^     In  1802  he  saw  seven  lines ;  the  five  most  promi- 


1  Consult  S.  Newoomb,  Astr.  Papers  for  the  Am.  Ephem.  and  Naut. 
Aim.,  Vol.  III.,  Part  III.,  1885. 

2  These  figures  and  some  other  details  have  been  taken  from  Preston, 
Theory  of  Light.,  Ch.  XIX.  For  a  fuller  account  of  researches  on 
light  the  reader  is  referred  to  R.  T.  Glazebrook,  "Report  on  Optical 
Tlieories,"  in  Beport  of  British  Association.,  1885,  abstracted  m  Nature., 
Vol,  48,  pp.  473-477  ;  Humphrey  Lloyd,  "Report  on  the  Progress  and 
Present  State  of  Physical  Optics,"  in  Beport  of  British  Association.,  1834. 

3  His  invention  of  the  process  of  rendering  platinum  malleable  brought 
him  a  considerable  annual  royalty.  He  invented  the  camera  lucida  and 
cryophorus  ;  he  discovered  palladium  and  rhodium. 


154  A   HISTORY    OF   PHYSICS 

nent  ones  were  considered  by  him  to  be  the  natural  boundaries 
or  dividing  lines  of  the  pure  simple  colours  of  the  spectrum.^ 
His  explanation  is  of  interest,  for  it  shows  how  a  most  plau- 
sible theory  may  be  destitute  of  all  truth.  Says  Wollaston : 
"...  The  colours  into  which  a  beam  of  white  light  is  sepa- 
rable by  refraction,  appear  to  me  to  be  neither  seven,  as  they 
usually  are  seen  in  the  rainbow,  nor  reducible  by  any  means 
(that  I  can  find)  to  three,  as  some  persons  have  conceived ;  but 
.  .  .  four  primary  divisions  of  the  prismatic  spectrum  may  be 
seen,  with  a  degree  of  distinctness  that,  I  believe,  has  not 
been  described  nor  observed  before."  ^ 

The  first  great  research  on  solar  dark  lines  was  made  by 
Fraunhofer,  who  had  no  knowledge  of  Wollaston's  discovery. 
Joseph  Fraunhofer  (1787-1826)  was  born  at  Straubing  in 
Bavaria.  He  was  the  son  of  a  poor  glazier,  and  early  in  life 
began  to  assist  his  father  in  his  trade.  Skilled  in  glass-grind- 
ing, he  secured  a  place  in  the  optical  institute  of  Utzschneider 
in  the  village  of  Benediktbeuern.  In  1818  he  took  charge  of 
the  institute,  which,  soon  after,  was  moved  to  Munich.     Fraun- 

1  Mrs.  Mart  Somerville,  the  mathematician  and  physicist,  gives  the 
following  recollections  :  "  One  bright  morning  Dr.  Wollaston  came  to  pay 
us  a  visit  in  Hanover  Square,  saying,  '  I  have  discovered  seven  dark  lines 
crossing  the  solar  spectrum,  which  I  wish  to  show  you ; '  then,  closing 
the  window  shutters  so  as  to  leave  only  a  narrow  line  of  light,  he  put  a 
small  glass  prism  into  my  hand,  telling  me  how  to  hold  it.  I  saw  them 
distinctly.  I  was  among  the  first,  if  not  the  very  first,  to  whom  he 
showed  these  lines,  which  were  the  origin  of  the  most  wonderful  series  of 
cosmical  discoveries,  and  have  proved  that  many  of  the  substances  of  our 
globe  are  also  constituents  of  the  sun,  the  stars,  and  even  the  nebulae. 
Dr.  Wollaston  gave  me  the  little  prism,  which  is  doubly  valuable,  being 
of  glass  manufactured  at  Munich  by  Fraunhofer,  whose  table  of  dark 
lines  has  now  become  the  standard  of  comparison  in  that  marvellous 
science,  the  work  of  many  illustrious  men,  brought  to  perfection  by 
Bunsen  and  Kirchhoff."  Personal  Becollections  of  Mary  Somerville^  by 
her  daughter  Martha  Somerville,  Boston,  1874,  p.  133. 

2  Philos.  Trans.,  1802,  p.  378. 


LIGHT  155 

hofer  became  a  member  of  the  Munich  Academy  of  Sciences, 
and  conservator  of  its  physical  cabinet.^ 

In  his  optical  work,  Fraunhofer  combined  to  a  rare  degree 
theoretic  insight  with  practical  skill.  "  By  his  invention  of 
new  and  improved  methods,  machinery,  and  measuring  instru- 
ments for  grinding  and  polishing  lenses,  by  his  having  the 
superintendence,  after  1811,  also  of  the  work  in  glass-melt- 
ing, enabling  him  to  produce  flint  and  crown  glass  in  larger 
pieces,  free  of  veins,  but  especially  by  his  discovery  of  a 
method  of  computing  accurately  the  forms  of  lenses,  he  has 
led  practical  optics  into  entirely  new  paths,  and  has  raised 
the  achromatic  telescope  to,  until  then,  undreamed-of  perfec- 
tion." ^ 

In  the  endeavour  to  determine  indices  of  refraction  of  glass 
for  particular  colours,  to  be  used  in  the  design  of  more  accurate 
achromatic  lenses,  Fraunhofer  accidentally  discovered  in  the 
spectrum  of  a  lamp  the  double  line  in  the  orange,  now  known 
as  the  sodium  line.  In  oil  and  tallow  light  and,  in  fact,  in  all 
firelight,  he  saw  this  sharply  defined,  bright,  double  line, 
"  exactly  in  the  same  place  and  consequently  very  useful "  in 
the  determination  of  indices.  A  ray  from  a  narrow  slit  was 
allowed  to  fall  upon  a  distant  flint-glass  prism,  placed  in  the 
position  of  least  deviation  in  front  of  the  telescope  of  a  theodo- 
lite. Fraunhofer  proceeded  to  use  sunlight.  "I  wished  to 
find  out,"  he  says,  "  whether  a  similar  bright  line  could  be 
seen  in  the  spectrum  of  sunlight  as  in  the  spectrum  of  lamp- 
light, and  I  found,  with  the  telescope,  instead  of  this,  an 
almost  countless  number  of  strong  and  feeble  vertical  lines, 
which,   however,  were   darker   than   the   other   parts   of   the 

1  ROSENBERGER,  III.,  p.   189. 

2  E.  LoMMEL  in  preface,  p.  vii.,  to  Joseph  von  Fraunhofer* s  Gesani' 
melte  Schriften,  Miinchen,  1888. 


156  A   HISTORY   OF   PHYSICS 

spectrum,  some  appearing  to  be  almost  perfectly  black." ^ 
On  examining  other  substances,  like  hydrogen,  alcohol,  sul- 
phur, he  found  the  bright  line  again.  This  must  have  been 
due,  of  course,  to  the  presence  of  sodium  as  an  impurity, 
the  minutest  quantity  of  which  will  exhibit  its  spectrum. 
Fraunhofer  examined  also  starlight,  and  recognized  in  Venus 
some  of  the  solar  lines.^ 

He  was  the  first  to  observe  spectra  due  to  gratings,  and 
with  them  he  made  the  earliest  determination  of  wave-lengths. 
His  gratings  were  of  wire  .04  to  .6  mm.  thick.  The  grating 
space  varied  from  .0528  to  .6866  mm.  He  made  ten  gratings 
and  found  the  wave-length  for  D  with  each.  The  results  ranged 
from  .0005882  to  .0005897,  giving  a  mean  value  of  .0005888 
mm.,  which  is  remarkably  accurate,  if  we  consider  the  crude- 
ness  of  his  gratings.^  A  paper  of  1823  contains  experiments 
with  two  glass  gratings  having  spaces  of  .0033  and  .0160  mm., 
respectively. 

Fraunhofer's  publication  of  1814  did  not  receive  prompt 
recognition,  nor  did  his  papers  of  1821  and  1823.  Physicists 
were  fighting  over  the  emission  and  wave  theories  of  light. 
The  attention  of  chemists  was  concentrated  upon  Dalton's 
atomic  theory  and  the  Berthollet-Proust  controversy  over  the 
law  of  definite  proportions.  The  full  explanation  of  the 
new  fact  brought   forth  by  Fraunhofer  was   not   given  for 

1  Gesammelte  Schriften,  op.  cit,  p.  10.  Quoted  from  the  memoir, 
"  Bestimmung  des  Brechungs-  und  des  rarbenzerstreiiungs-Vermogens 
verschiedener  Glasarten,  in  Bezug  auf  die  Vervollkommnung  achroma- 
tischer  rernrohre,"  which  appeared  first  in  Denkschriften  der  Munchener 
Akad.,  Band  V.,  1814-1815. 

2  G.  W.  A.  Kahlbaum,  Aus  der  Vorgeschichte  der  Spectralanalyse, 
Basel,  1888,  p.  12. 

3  See  Fraunhofer,  Neue  Modification  des  Lichtes,  1821  ;  also  Louis 
Bell,  "The  Absolute  Wave-Length  of  Light"  in  Fhilos.  Magazine  (5), 
Vol.  25,  1888,  p.  245. 


LIGHT  157 

nearly  forty  years.  He  himself  had  failed  to  find  the  key 
to  the  hieroglyphics  of  the  solar  lines,  the  "  Fraunhofer  lines/' 
nor  had  he  clearly  defined  the  role  which  the  spectral  lines 
were  destined  to  play  in  chemical  analysis. 

After  Fraimhofer,  the  first  researches  were  made  in  Eng- 
land. J.  F.  W.  Herschel  examined  bright-line  spectra  of  sev- 
eral substances,  stated  that  the  colours  of  the  bright  lines  were 
a  means  of  detecting  small  quantities  of  a  substance,  and  in 
1827  touched  on  this  subject  in  his  work  On  Light.  Charles 
Wlieatstone  published,  in  1835,  a  paper  on  spectra  of  the 
electric  arc  passing  between  metals.  Williain  Henry  Fox 
Talbot  (1800-1877),  a  rich  citizen,  expressed  the  belief  that 
every  homogeneous  ray,  whatever  its  colour,  always  indicates 
the  presence  of  a  definite  chemical  compound.  Yet  none  of 
these  investigators  arrived  at  clear  notions  on  the  subject. 
Talbot,  for  instance,  falls  into  an  error  which  inexperienced 
students  in  our  laboratories  frequently  commit :  he  looks  upon 
certain  bright-line  spectra  as  being  really  dark-line  spectra. 
"  Copper-salts  give  spectra  so  covered  with  dark  lines  as  to 
resemble  the  solar  spectrum.^'  ^  Kirchhoff  points  out  that  the 
English  investigators  did  not  establish  the  strict  dependence 
of  the  spectral  lines  upon  the  particular  element  in  the  flame ;  ^ 
thus  Talbot  ascribes  the  D  line  to  both  sulphur  and  the  salts 
of  sodium.  Sir  David  Brewster,  in  1832,  described  dark-line 
spectra,  formed  by  absorption  of  rays  passing  through  coloured 
glass  and  through  certain  gases.  These  spectra  simulated  the 
solar  spectrum.  In  the  fact  that  fuming  nitric  acid  absorbs 
lines,  while  the  liquid  does  not,  Brewster  saw  an  argument 
against  the  wave  theory  of  light ;  for  a  gas  ought  to  offer  less 
impedance  to  motion  of  the  ether  than  its  denser  liquid.     The 

1  Kahlbaum,  op.  cit. ,  p.  18. 

2  G.  KiRCHHorr,  "Zur  Geschichte  der  Spectralanalyse,"  Gesammelte 
Abhandlungen,  Leipzig,  1882,  pp.  625-641 ;  Rosenberger,  III.,  p.  313 


158  A   HISTORY   OF   PHYSICS 

exact  coincidence  of  the  bright  lines  of  sodium  with  the  dark 
D  lines  of  the  sun  was  established  by  William  Allen  Miller  of 
Kings  College,  and  by  FoucauU  in  Paris.  The  latter  did  this 
by  introducing  simultaneously  into  the  spectroscope  sunlight 
and  the  electric  light  displaying  the  sodium  lines.  The  pos- 
sible production  of  the  Eraunhofer  lines  through  absorption  of 
certain  rays  by  the  solar  atmosphere  was  then  under  considera- 
tion, but  no  definite  conclusion  was  reached  as  to  the  validity 
of  this  explanation. 

A  great  aid  to  the  study  of  spectra  was  the  discovery  of  the 
art  of  photography  by  Joseph  Nicephore  Niepce  (1765-1833), 
who  produced  photographic  pictures  on  metal  in  1827.  Louis 
Jacques  Mande  Daguerre  (1789-1851)  was  for  some  years 
Niepce's  coadjutor,  and  subsequently  improved  the  method  of 
the  latter,  announcing  in  1839  the  new  process  known  as  the 
"daguerreotype."  This  famous  process  was  at  once  taken  up  by 
J.  W.  Draper  in  New  York,  who  was  the  first  to  apply  it  to 
individuals.  In  the  first  trials,  "  the  face  of  the  sitter  .  .  . 
was  dusted  with  white  powder,"  and  on  a  bright  day  a  picture 
was  taken  in  five  or  seven  minutes.  In  1840  Draper  pho- 
tographed the  moon;  in  1842  he  photographed  the  Eraun- 
hofer  lines,  only  a  few  months  after  a  similar  achievement 
by  Edmond  Becquerel  in  Erance.  In  1843  Joseph  Saxton,  a 
mechanician  of  the  United  States  mint  in  Philadelphia,  ruled 
for  Draper  a  diffraction  grating  of  glass,  and  the  latter  photo- 
graphed the  diffraction  spectrum.  We  will  now  sketch  the 
life  of  this  assiduous  investigator. 

John  William  Draper  (1811-1882)  was  born  at  St.  Helen's, 
near  Liverpool,  and  studied  at  the  London  University.  He 
came  to  the  United  States  in  1833.  After  studying  medicine 
at  the  University  of  Pennsylvania,  he  was  chosen  to  the  chair 
of  chemistry  and  physiology  at  Hampden-Sidney  College,  Vir- 
ginia, and  later  to  the  same  chair  at  the  University  of  New 


LIGHT  159 

York,  where  he  remained  until  the  end  of  his  long  life.  For 
many  years  he  dwelt  in  a  quiet  retreat  at  Hastings-on-the-Hud- 
son,  near  New  York,  surrounded  by  everything  which  could 
minister  to  the  tastes  of  a  veteran  in  science/ 

In  1847,  Draper  published  an  important  memoir,^  in  which 
he  concluded  from  experiment  that  all  solid  substances 
and  probably  liquids  become  incandescent  at  the  same  tem- 
perature, viz.,  red  hot  at  525°  C. ;  that  below  525°  C.  invis- 
ible rays  are  emitted,  and  as  the  temperature  rises  above 
525°,  rays  of  greater  refrangibility  are  added  successively  and 
continuously ;  that  all  spectra  of  incandescent  solids  are  con- 
tinuous, that  gases  give  continuous  spectra  too,  but  may  have 
bright  lines  superposed.  The  last  statement  is  incorrect.  The 
error  originated  in  his  use  of  bright  flames  giving,  in  addition 
to  the  line  spectrum  of  the  salt  placed  in  the  flame,  the  con- 
tinuous spectrum  of  solid  carbon;  a  luminous  gas  ordinarily 
gives  only  bright  lines. 

Thirteen  years  later  Draper's  correct  conclusions  were 
deduced  independently  from  theoretical  considerations  by 
Kirchhoff,  who  started  out  from  the  relation  between  emitting 
and  absorbing  powers  possessed  by  different  bodies  for  radiant 
energy.      This    relation   had    been    established    in    1854    by 

o 

Angstrom  (and  later  by  Balfour  Stewart). 

An  exhaustive  account  of  spectrum  analysis  before  Kirchhoff 
and  Bunsen  would  call  for  further  reference  to  researches  made 

o 

by  Andreas,  Ayigstrom,  Balfov.r  Stewart,  Sir  David  Breivster, 
J.  H.  Gladstone,   Julius  PUlcker   (the   inventor  of   "Plticker 

1  Am.  Jour,  of  Science  (3),  Vol.  23,  1882,  p.  163 ;  see  also  JSFat  Acad, 
of  Sciences,  Biographical  Memoirs,  Vol.  II.,  1886,  p.  351. 

2  Philos.  3Iagazine,  May,  1847  ;  J.  W.  Draper's  Scientific  Memoirs, 
New  York,  1878,  "Memoir  I."  ;  see  also  J.  W.  Draper,  "Early  Con- 
tributions to  Spectrum  Photography  and  Photo-Chemistry,"  Nature,  Vol 
X.,  1874. 


160  A   HISTORY    OF    PHYSICS 

tubes "),   V.  S.  M.  van  der  Willigen,  Edmond  Becquerel,  and 
many  others.-^ 

Gustav  KirchJioff  (1824-1887)  was  born  at  Konigsberg;  he 
became  privat-docent  in  Berlin,  then  extraordinary  professor 
at  Breslau,  ordinary  professor  at  Heidelberg  in  1854,  and 
professor  in  Berlin  after  1875.  The  rich  period  of  his  life  was 
the  twenty  years  he  taught  at  Heidelberg,  where  he  worked 
conjointly  with  the  great  chemist,  Bobert  WiUielm  Bunsen 
(born  1811).^  It  was  during  the  years  1859-1862  that  these 
great  investigators  together  made  the  great  discoveries  of  spec- 
trum analysis.  At  that  time  the  physical  laboratory  at  Heidel- 
berg was  very  unpretentious,  being  located  in  a  house,  the 
"  Riesengebaude,"  then  150  years  old.  The  memorable  re- 
searches were  carried  on  in  a  small  room.  Illuminating  gas 
had  been  introduced  into  the  building  in  1855.^  In  1857 
Bunsen  and  Roscoe  first  described  the  "Bunsen  burner."^ 
This  new  burner  furnished  Bunsen  and  Kirchhoff  with  a 
non-luminous  gas-flame  of  fairly  high  temperature,  in  which 
chemical  substances  could  be  vaporized  and  a  spectrum  could 
be  obtained,  due  purely  and  simply  to  the  luminous  vapour. 
In  this  way  some  of  the  errors  of  earlier  experimenters  were 
avoided. 

In  October,  1859,  Kirchhoff  and  Bunsen  published  their  first 
paper,^  which  contains  their  later  researches  in  mice.  From 
experiments  the  conclusion  is  drawn  by  Kirchhoff  "that  a 
coloured  flame,  the  spectrum  of  which  contains  bright  sharp 

1  Consult  Kahlbaum,  op.  cit. ;  Kirchhoff,  "Zur  Geschichte  der 
Spectralanalyse." 

2  For  his  contributions  to  chemistry,  see  Nature,  Vol.  2.3,  1881,  p.  597. 

*  Georg  Quincke,  Gesch.  d.  Physik.  Instituts  d.  Univ.  Heidelberg, 
Heidelberg,  1885,  p.  16. 

*  Poggendorff^s  Annalen,  C,  pp.  84-86 ;  Kosenberger,  III.,  p.  484. 

5  "Ueber  die  Fraunhoferschen  Linien,"  in  Monatsberichte  d.  Akad.  d. 
Wissensch.  zu  Berlin,  October,  1859,  p.  662. 


LIGHT  161 

lines,  so  weakens  rays  of  tlie  colour  of  these  lines,  wlien  tliey 
pass  throiigli  it,  that  dark  lines  appear  in  place  of  the  bright 
lines  as  soon  as  there  is  placed  behind  the  flame  a  light  of 
sufficient  intensity,  in  which  the  lines  are  otherwise  absent ; " 
"that  the  dark  lines  of  the  solar  spectrum,  which  are  not 
caused  by  the  terrestrial  atmosphere,  arise  from  the  presence 
in  the  glowing  solar  atmosphere  of  those  substances  which  in 
a  flame  produce  bright  lines  in  the  same  position."  Kirchhoff 
concluded  that  sodium,  iron,  magnesium,  copper,  zinc,  barium, 
nickel,  existed  in  the  solar  atmosphere. 

The  two  investigators  advanced,  as  scientifically  established, 
the  law  that  the  bright  lines  in  the  spectrum  may  be  taken  as 
a  sure  sign  of  the  presence  of  the  respective  metals.  This 
conclusion  was  rendered  doubly  sure  by  the  discovery  in  the 
mineral  water  at  Dtirkheim,  through  the  spectrum,  of  two  new 
metals.  From  the  blue  and  the  red  lines,  by  which  they  were 
recognized,  they  were  named  "  Caesium  '^  and  "  Eubidium." 
While  spectrum  analysis,  as  a  terrestrial  science,  was  due 
equally  to  Kirchhoff  and  Bunsen,  its  celestial  applications  be- 
long to  Kirchhoff  alone.  Kirchhoff's  explanation  of  the  Fraun- 
hof er  lines  was  epoch-making.  Says  Helmholtz : ^  "It  had  in 
fact  most  extraordinary  consequences  of  the  most  palpable 
kind,  and  has  become  of  the  highest  importance  for  all 
branches  of  natural  science.  It  has  excited  the  admiration 
and  stimulated  the  fancy  of  men  as  hardly  any  other  dis- 
covery has  done,  because  it  has  permitted  an  insight  into 
worlds  that  seemed  forever  veiled  for  us."  In  this  connection 
Kirchhoff  frequently  related  the  following  story :  ^  "  The 
question  whether  Fraunhofer's  lines  reveal  the  presence  of 

1  "A  Memoir  of  Gustav  Robert  Kirclihoff,"  Deutsche  Bundschau,  Feb- 
ruary 1888,  Vol.  14,  pp.  232-245  ;  translated  in  Smithsonian  Beport, 
1889,  pp.  527-540. 

2  Smithsonian  Beport,  1889,  p.  537. 


162  A  HJ3X0RY  OF  PHYSICS 

gold  in  the  sun  was  being  investigated.  KirchliofE's  banker 
remarked  on  this  occasion :  '  What  do  I  care  for  gold  in  the 
sun  if  I  cannot  fetch  it  down  here  ? '  Shortly  afterwards 
Kirchhoff  received  from  England  a  medal  for  his  discovery, 
and  its  value  in  gold.  While  handing  it  over  to  his  banker, 
he  observed,  ^  Look  here,  I  have  succeeded  at  last  in  fetching 
some  gold  from  the  sun.' " 

It  has  been  said  that  Kirchhoff's  gift  as  an  investigator  was 
not  to  initiate,  but  to  complete}  This  is  plainly  seen  in  his 
work  on  spectrum  analysis.  The  threads  of  his  discovery  had 
been  seized  upon  by  great  men  before  him.  So  nearly  had 
English,  French,  and  American  scientists  attained  to  Kirch- 
hoff's results,  that  prolonged  discussions  liave  arisen  on  ques- 
tions of  priority.  "All  had  seen  something,  made  guesses, 
considered  as  possible  or  probable  (without  Kirchhoff  having 
been  aware  of  it  at  the  time,  however)."  But  it  remains  the 
great  merit  of  Kirchhoff  to  have  established  a  solid  basis,  to 
have  arrived  at  sure  knowledge. 

One  claim  of  priority  was  made  in  favour  of  William  Hallows 
Miller  of  Cambridge,  who,  it  was  argued,  "anticipated  by 
nearly  sixteen  years  the  remarkable  discovery,  ascribed  to 
Kirchhoff,  of  the  opacity  of  certain  coloured  flames  to  light  of 
their  own  colour."  ^  Another  claim  was  made  soon  after  Kirch- 
hoff's paper  of  1859  by  William  Thomson  (now  Lord  Kelvin) 
in  favour  of  George  Gabriel  StoTies  (born  1819)  of  Pembroke 
College,  Cambridge,  who,  before  Kirchhoff  (perhaps  about  the 
year  1849),  in  course  of  a  conversation,  explained  the  forma- 
tion of  absorption  lines  as  follows :  "  Vapour  of  sodium  must 
possess  by  its  molecular  structure  a  tendency  to  vibrate  in  the 

1  W.  YoiGT,  Zum  Geddchtniss  von  G.  Kiixhhoff,  Gottingen,  1888, 
p.  9. 

2  Crookes  in  Chemical  News^  May  18,  1862  ;  Philos.  Magazine  (4), 
Vol.  25,  1863,  p.  261. 


LIGHT  163 

periods  corresponding  to  tlie  degree  of  refrangibility  of  the 
double  line  D.  Hence  the  presence  of  sodium  in  a  source  of 
light  must  tend  to  originate  light  of  that  quality.  On  the 
other  hand,  vapour  of  sodium  in  an  atmosphere  round  a  source 
must  have  a  great  tendency  to  retain  itself,  i.e.  to  absorb  and 
have  its  temperature  raised  by  light  from  the  source  of  the 
precise  quality  in  question.  In  the  atmosphere  round  the  sun, 
therefore,  there  must  be  present  vapour  of  sodium,  which,  ac- 
cording to  the  mechanical  explanation  thus  suggested,  being 
particularly  opaque  for  light  of  that  quality,  prevents  such  of 
it  as  is  emitted  from  the  sun  from  penetrating  to  any  con- 
siderable distance  through  the  surrounding  atmosphere."^ 
Stokes  did  not  ascertain  experimentally  whether  or  not  the 
vapour  of  sodium  has  the  special  absorbing  power  anticipated, 
but  he  remembered  a  test,  showing  this  power,  made  in  France 
by  Poucault.^  He  did  not  attach  sufficient  importance  to  his 
mechanical  theory  to  have  it  appear  in  print.  Sir  William 
Thomson,  however,  adds  this:  "I  have  given  it  in  my  lectures 
regularly  for  many  years,  always  pointing  out  along  with  it 
that  solar  and  stellar  chemistry  were  to  be  studied  by  investi- 
gating terrestrial  substances  giving  bright  lines  in  the  spectra 
of  artificial  flames  corresponding  to  the  dark  lines  of  the  solar 
and  stellar  spectra."  Stokes  himself  generously  published  the 
following  disclaimer:  "I  have  never  attempted  to  claim  for 
myself  any  part  of  Kirchhoff's  admirable  discovery,  and  can- 
not help  thinking  that  some  of  my  friends  have  been  over 
zealous  in  my  cause."  ^ 

Since  the  creation  of  the  science  of  spectrum  analysis  by 
Kirchhoff  and  Bun  sen,  scientists  have  been  busy  perfecting 
the  details  of  the  theory,  improving  methods  of  experimental 

1  PMlos.  Magazine  (4),  Vol.  25,  1863,  p.  261. 

2  L' Institute  Feb.  7,  1849,  p.  45.   ^ 
8  Nature,  Vol.  13,  1875,  p.  189. 


164  A   HISTORY   OF  PHYSICS 

tioHj  and  enlarging  our  knowledge  of  celestial  chemistry.  It 
soon  became  evident  that  great  caution  must  be  exercised  in 
deducing  the  chemical  constitution  and  physical  characteristics 
of  bodies  from  the  spectra  which  they  give.  Confusion  is 
introduced  by  the  occurrence  of  multiple  spectra.  As  early  as 
1862,  Julius  Plucker,  in  Bonn,  pointed  out  that  the  same  sub- 
stance may  give  different  spectra  at  different  temperatures. 
He  and  TF.  Hittorf  found  for  hydrogen,  nitrogen,  and  sulphur 
fumes  two  kinds  of  spectra,  namely,  a  weak  band  spectrum 
and  a  bright  line  spectrum.  Adolpli  Wullner  of  the  Technicum 
in  Aachen,  in  1868,  discussed  the  variation  in  the  spectra  of 
hydrogen,  oxygen,  nitrogen,  when  subjected  in  Plucker  tubes 
to  different  degrees  of  pressure.^  For  oxygen  he  observed 
three  spectra  under  different  conditions  of  pressure.  As  in  a 
denser  gas  the  electric  resistance  to  the  discharge  through  the 
tube  was  greater,  the  temperature  was  probably  higher. 
Hence  Wullner  thought  that  in  Plucker  tubes  variations  in 
pressure  of  the  gas  were  accompanied  by  changes  in  the 
temperature,  and   that   the    spectral   changes   resulted    from 

o 

alterations  in  both  pressure  and  temperature.  Angstrom 
combated  Wlillner's  position,  arguing  that  while  a  rise  in 
temperature  may  bring  out  new  lines  and  an  increase  in 
pressure  may  widen  the  lines,  nevertheless  a  spectrum  never 
changes  into  another  of  entirely  new  characteristics.^     Some 

o 

of  Wlillner's  results  were  attributed  by  Angstrom  to  the 
presence  of  impurities  in  the  gases.  However,  more  extended 
research  revealed  that  spectral  changes  depend  not  only 
upon  variations  in  temperature  and  pressure,  but  also  upon 
molecular  constitution.  The  effect  of  molecular  structure  was 
investigated  by  Al.  Mitsclierlich,  Clifton,  H.  E.  Boscoe,  and  by 

^  Poggendorff''s  Annalen,  Vol.  135,  p.  497. 

2  Becherches  sur  le  Spectre  Solaire,  Upsala,  1868.     See  Eosenberger, 
III.,  p.  701. 


LIGHT  165 

J.  Norman  Lockyer}  Lockyer,  in  1873  and  1874,  advanced  the 
view  that  each  composite  body  has  as  definite  a  spectrum  as  a 
simple  one;  that  line  spectra  are  due  to  the  free  atoms,  band 
spectra  to  molecules  or  groups  of  molecules.     Lockyer's  theory 

o 

was  regarded  favourably  by  Angstrom,  but  was  opposed  by 
Wtillner,  who  in  1879  ^  made  experiments  on  nitrogen,  showing 
that  by  gradual  change  of  temperature  the  band  spectra  passed 
gradually  into  the  line  spectra.  He  argued  that  Lockyer's 
theory  of  the  dissociation  of  molecules  was  not  needed  to 
explain  the  facts.  Lockyer  observed  that  line  spectra  (of 
calcium,  for  instance)  change  as  the  temperature  rises.  He 
then  advanced  the  bold  theory  that  just  as  the  transition  of 
band  spectra  into  line  spectra  may  be  explained  by  the  dis- 
sociation of  molecules  into  atoms,  so  the  changes  in  the  line 
spectra,  due  to  rise  in  temperature,  may  be  explained  by  the 
breaking  up  of  the  atoms  into  still  more  elementary  substances, 
thus  indicating  the  compound  nature  of  the  chemical  elements 
themselves.^ 

The  Germans,  H.  Kayser  and  C.  Bunge,  in  a  series  of 
researches,  beginning  in  1890,  have  shown  that  the  distribu- 
tion of  lines  in  the  spectra  of  the  elements  is  by  no  means  so 
irregular  as  it  at  first  seems.  They  find  that  in  the  spectra  of 
the  common  elements  there  are  line  series.  At  one  time  the 
presence  in  argon  of  more  than  one  series  was  supposed  to  indi- 
cate that  it  was  a  mixture  of  elements ;  but  as  the  same  reason- 
ing applied  to  oxygen,  which  has  six  series,  leads  to  conclusions 
presumably  erroneous,  this  hypothesis  has  been  abandoned. 

1  Consult  J.  N.  Lockyer,  Studies  in  Spectrum  Analysis,  New  York, 
189.3,  Chap.  VII. 

2  EosENBERGER,  III,,  p.  706.  Consult  report  "On  the  Present  State 
of  Spectrum  Analysis,"  Beport  of  Brit.  Ass.,  Swansea  meeting,  1880; 
abstracted  in  Nature,  Vol.  22,  p.  522. 

*  Lockyer,  op.  cit.,  p.  189. 


166  A   HISTOBY   OF   PHYSICS 

It  is  still  doubtful  whether  increased  pressure  augments  the 
breadth  of  lines.  G.  D.  Liveing  and  J.  Deivar  have  combated 
the  theory  that  the  continuous  spectra  are  produced  by  the 
broadening  of  the  lines  of  the  same  gas  at  low  pressure.^  An 
important  observation  was  made  in  1895  by  W.  J.  Humphreys 
and  J.  F.  Moliler  in  the  Johns  Hopkins  University  laborer 
tory.  Certain  discrepancies  noticed  by  L.  E.  Jewell  led  them 
to  undertake  experiments  which  demonstrate  that  the  lines 
in  the  arc  spectra  of  metals  shift  appreciably  toward  the  red 
when  the  pressure  of  the  atmosphere  surrounding  the  arc  is 
increased.  This  may  be  distinguished  from  the  Doppler  effect 
by  the  fact  that  the  displacement  is  different  for  every  metal 
and  for  different  spectral  series  of  the  same  metal.^  Another 
interesting  phenomenon,  showing  the  influence  of  magnetization 
on  light,  was  observed  in  1896  by  P.  Zeeman,  now  professor  at 
the  University  of  Amsterdam.  In  1862  Faraday  had  examined 
the  sodium  lines  when  the  flame  was  placed  between  the  poles 
of  a  magnet,  but  had  failed  to  notice  any  effect ;  Zeeman,  by 
means  of  modern  appliances,  noticed  a  change.  Light  from  an 
electric  arc  was  sent  through  a  heated  tube  containing  sodium 
vapour  and  placed  between  the  poles  of  an  electro-magnet. 
"When  acted  upon  by  the  magnet  a  slight  broadening  of  the 
lines  was  seen.^  A.  A.  Michelson  of  the  University  of  Chicago, 
using  his  new  echelon  spectroscope,  showed  that  the  phenome- 
non is  much  more  complex.  For  instance,  "  all  spectral  lines 
are  tripled  when  the  radiations  emanate  in  a  magnetic  field." 

The  spectroscope  has  been  used  extensively  in  the  chemical 

1  W.  HuGGiNs,  Inaugural  Address,  Nature,  Vol.  44,  1891,  p.  373. 

2  Astrophys.  Jour.,  Vol.  III.,  1896,  pp.  114-137  ;  Johns  Hopkins  Univ. 
Circular,  No.  130 ;  Nature,  Vol.  56,  1897,  pp.  415,  461. 

3  P.  Zeeman  in  Phil.  Mag.,  Vol.  43,  pp.  226-239 ;  Nature,  Vol.  55,  pp. 
192,  347  ;  consult  0.  Lodge  in  Electrician  (London),  Vol.  38,  pp.  568, 
643. 


LIGHT  167 

analysis  of  heavenly  bodies/  but  it  has  received  also  an 
indirect  application,  which  promises  to  become  hardly  less 
important.  A  telescope  gives  us  no  direct  evidence  of  stellar 
motion  in  a  direction  toward  us  or  from  us,  but  now  the 
spectroscope  has  placed  in  our  hands  the  means  of  detecting 
such  motion.  The  principle  involved  was  first  worked  out  for 
sound  by  Johann  Christian  Doppler  (1803-1853),  a  native  of 
Salzburg,  Austria.  In  1835,  having  been  unable  to  secure  a 
suitable  situation,  he  was  about  to  emigrate  to  America,  when 
he  was  made  professor  of  mathematics  at  the  E-ealschule  in 
Prague.^  He  called  attention,  in  a  paper  of  1842,  to  the  fact 
that  the  colour  of  a  luminous  body,  just  like  the  pitch  of  a 
sounding  body,  must  be  changed  by  motion  of  the  body  to  or 
from  the  observer.  In  the  year  1845,  Christoph  Heinrich 
Dietrich  Buys-Ballot  (born  1817),  director  of  the  royal  meteoro- 
logical institute  at  Utrecht,  experimented  on  railroad  trains, 
and  verified  the  theory  as  applied  to  sound.  A  person  on  a 
train  rushing  through  a  station  finds  the  pitch  of  a  sounding 
bell  at  the  station  higher  on  approach  and  lower  on  recession 
than  it  actually  is.  Doppler  argued  that  most  probably  all 
stars  emitted  white  light,  and  that  the  colour  of  some  of  them 
was  due  to  their  motion  toward  us  or  away  from  us.  As  Buys- 
Ballot  pointed  out,  this  conclusion  is  erroneous.  The  approach 
of  a  star  would  simply  produce  a  slight  shift  of  the  entire 
spectrum  in  the  direction  of  the  ultra-violet  region,  some  infra- 
red rays  becoming  visible  and  some  violet  rays  becoming 
invisible.     No  change  in  colour  could  take  place.     But  in  1848 

1  For  the  history  of  astrophysics  consult  A.  M.  Clerke,  History  of 
Astronomy  during  the  Nineteenth  Century.  Eor  "Literature  of  the 
Spectroscope,"  see  Smithsonian  Miscellaneous  Collections^  Vol.  32,  1888. 

2  Before  his  death  he  was  professor  of  experimental  physics  at  the 
University  of  Vienna.  See  E.  Poske,  Zeitsch.  f.  d.  Physik.  u.  Chem, 
Unterricht,  Vol.  9,  1896,  p.  248. 


£68  A    HISTORY    OF   PHYSICS 

Fizeau  pointed  out  that  this  shifting  must  become  noticeable 
through  the  examination  of  the  lines  of  the  spectrum.  For 
instance,  if  the  hydrogen  lines  of  an  approaching  star  are  com- 
pared with  those  of  a  hydrogen  tube  in  the  laboratory,  the 
former  are  moved  toward  the  violet,  while  the  latter  are  fixed. 
The  displacement  is  so  slight  that  many  years  elapsed  before 
instruments  were  devised  by  which  accurate  measurements 
could  be  taken.  The  initiative  in  this  delicate  work  was  taken 
in  1868  by  the  English  astronomer,  William  Huggins  (born 
1824),  and,  in  1871,  H.  C.  Vogel  of  Potsdam  detected  the  shift- 
ing effects  due  to  the  sun's  rotation.  In  recent  years  Doppler's 
principle  has  been  applied  with  great  success  to  the  motions  of 
stars  and  to  the  discovery  of  double  stars  by  Vogel,  Edward  C. 
Pickering  of  Harvard,  James  E.  Keeler  of  the  Lick  Observa- 
tory, and  others.  Some  double  stars  discovered  by  this  method 
are  so  close  to  each  other  that  they  appear  like  a  single  star 
even  when  examined  by  our  most  powerful  telescopes. 

There  are  two  methods  of  obtaining  spectra :  one  is  by  the 
aid  of  a  prism  or  a  train  of  prisms,  the  other  by  the  use  of  a 
grating.  The  former  means  was  employed  by  Kirchhoff  and 
Bunsen;  the  latter  was  used  to  some  extent  by  Fraunhofer 
and  by  J.  W.  Draper.  Tlie  theory  of  the  grating  ("  striated 
surfaces")  had  been  outlined  by  Thomas  Young.  After 
Fraunhofer  the  first  improvement  in  the  art  of  manufacturing 
gratings  was  made  by  the  optician,  F.  A.  Nohert,  of  Greifswald 
in  Pomerania.  He  made  glass  micrometers,  which  Avere  used 
to  determine  the  magnifying  power  of  microscopes,  and  he  fur- 

J  o 

nished  gratings  to  E.  Mascart  and  Angstrom.  The  latter  pub- 
lished at  Upsala,  in  1868,  in  his  Recherches  sur  le  Spectre 
Solaire,  a  table  of  wave-lengths  which  for  a  long  time  served 
as  a  standard.  All  the  measurements  are  in  error  by  about 
one  part  in  seven  or  eight  thousand,  owing  mainly  to  the  fact 
that  the  metre  which  he  used  as  the  standard  of  length  was  a 


LIGHT  169 

o 

trifle  too  short.^  Angstrom  became  aware  of  this  as  early  as 
1872,  but  he  did  not  live  to  make  the  needed  alterations.  The 
corrections  were  made  by  his  pnpil,  R.  TliaUn,  in  a  publication 
of  1885. 

Nobert's  method  of  ruling  diffraction  gratings  was  jealously 
guarded  by  him  as  a  trade  secret.  Since  his  time  the  best 
gratings  have  been  made  in  the  United  States.  About  1863 
Lewis  Morris  ButJierfurd  (1816-1892),  a  graduate  of  Williams 
College,  and  a  lawyer,  who  studied  astronomy  in  his  own  private 
observatory  near  New  York,  became  interested  in  the  prepara- 
tion of  gratings.  Kutherfurd,  after  numerous  preliminary 
experiments,  constructed  a  machine  of  his  own  device,  and  ran 
it  by  means  of  a  small  water  motor.  "  A  diamond  point  traced 
parallel  lines  upon  a  glass  plate  pushed  regularly  forward  by  a 
system  of  levers  acting  on  an  acute  glass  wedge,  this  in  its 
turn  pushing  the  plate  sideways."  ^  Except  for  occasional 
slight  changes  in  the  intervals  between  the  lines  the  gratings 
were  admirable.  Following  the  advice  of  Ogden  JSf.  Rood  of 
Columbia  College,  he  constructed,  in  1867,  a  machine  in  which 
the  plate  was  moved  by  a  screw  in  place  of  levers.  After 
several  years'  effort  he  produced  gratings  far  superior  to 
JSTobert's.  In  1875,  or  earlier,  E-utherfurd  silvered  the  gratings 
with  the  view  to  their  more  convenient  spectroscopic  use,  but 
later  he  made  gratings  upon  speculum  metal  in  order  to  avoid 
the  great  wear  upon  the  diamond.^  In  1877  the  ruling  machine 
was  enlarged.  Armed  with  Eutherfurd's  superior  gratings, 
Charles   Saunders  Peirce,   then   of   the   United   States   Coast 

iL.  Bell,  "The  Absolute  Wave-Length  of  Light,"  Phil.  Mag.  (5), 
Vol.  25,  1888,  p.  245. 

2  B.  A.  Gould  in  Nat.  Acad,  of  Sciences^  Biographical  Memoirs, 
Vol.  3,  p.  428. 

3  For  details,  consult  article,  "Ruling  Machines,"  in  Johnson's  Uni- 
versal Cyclopcedia. 


170  A   HISTORY   OF   PHYSICS 

Survey,  again  attacked  the  problem  of  wave-lengths  where 
Angstrom  had  left  it  ten  years  previously.^ 

The  best  gratings  of  the  present  time  are  those  of  Henry  A. 
Rowland  of  the  Johns  Hopkins  University.  His  attention 
was  first  called  to  the  construction  of  dividing  engines  by  the 
inspection  of  an  engine  made  by  William  Augustus  Rogers 
(1832-1898),  at  Waltham,  Mass.^  Eogers's  aim  was  to  produce 
lines  of  extreme  fineness  for  recticules  in  optical  instruments, 
and  for  delicate  tests  of  microscope  objectives.  He  was  able 
to  rule  as  many  as  4800  lines  to  the  millimetre.  Rowland 
devoted  about  one  3^ear  to  the  construction  of  a  dividing 
engine.  The  making  of  an  accurate  screw  was  the  most  deli- 
cate part  of  the  task.  The  process  consisted  in  grinding 
the  screw  in  a  long  nut  in  which  it  was  constantly  reversed. 
When  it  was  finished,  there  was  not  an  error  of  half  a  wave- 
length, although  it  was  nine  inches  long.^  Rowland  invented 
concave  gratings,  and  ruled  them  on  his  engine.  The  colli- 
mator could  thereby  be  dispensed  with.  A  second  and  a  third 
engine  have  been  prepared  under  Rowland's  direction,  and  at 
present  Rowland's  gratings  have  no  rival.'*  He  has  made  large 
photographic  maps  of  the  solar  spectrum,  and  has  prepared  a 
system  of  standard  wave-lengths  which  is  now  universally 
adopted.  Under  his  direction,  the  wave-length  of  every  line 
in  the  solar  spectrum  is  being  measured,  and  the  chemical 
element  to  which  it  belongs  is  being  determined. 

That  the  solar  spectrum  is  not  confined  to  the  visible  part, 

1  Am.  Jour.  Sci.  (3),  Vol.  18,  1879,  p.  51. 

2  Proc.  Am.  Acad,  of  Arts  and  Sci.,  New  Series,  Vol.  II.,  1883-1884, 
p.  482. 

3  Consult  Rowland's  article,  "Screw,"  in  Encyclopcedia  Britannica, 
9th  ed. 

*  Eor  a  biographical  sketch  of  Rowland,  and  a  picture  of  his  second 
dividing  engine,  see  Appleton^s  Pop.  Sci.  Month.,  Vol.  49,  1896, 
pp.  110-120. 


LIGHT  171 

extending  from  the  red  to  the  violet,  was  first  shown  by  Sir 
William  Herschel  (1738-1822),  who  in  1800  discovered  that 
there  are  infra-red  solar  rays.  Placing  the  thermometer  in 
successive  colours,  he  discovered  the  unequal  distribution  of 
heat  in  the  solar  spectrum,  the  heating  being  greatest  below 
the  red.  Before  him  no  one  had  suspected  such  an  inequality. 
"  It  is  sometimes  of  great  use  in  natural  philosophy,"  says  the 
veteran  astronomer,^  "to  doubt  of  things  that  are  commonly 
taken  for  granted ;  especially  as  the  means  of  resolving  any 
doubt,  when  once  it  is  entertained,  are  often  within  our  reach." 
He  speaks  of  solar  heat  as  occasioned  by  "-'rays,"  subject  to 
the  laws  of  reflection  and  refraction."  Thomas  Young,  in  his 
Lectures  of  1807,  says,  "  This  discovery  must  be  allowed  to  be 
one  of  the  greatest  that  has  been  made  since  the  days  of 
Newton."  Nevertheless,  the  mass  of  physicists  and  text-book 
writers,  for  over  half  a  century,  failed  to  see  the  truth  fore- 
shadowed by  W.  Herschel,  and  afterwards  established  more 
clearly  by  Melloni.  Herschel's  views  were  attacked  by  John 
Leslie  (1766-1832)  of  Edinburgh,  the  inventor  of  the  differen- 
tial thermometer.  This  able  and  earnest  investigator,  like  all 
seekers  after  the  truth,  fell  into  error.  He  saw  no  affinity 
between  radiant  heat  and  light.  He  says:  "What,  then,  is 
this  calorific  and  f rigorific  fluid  after  which  we  are  inquiring  ? 
It  is  no  light,  it  has  no  relation  to  ether,  it  bears  no  analogy 
to  the  fluids,  real  or  imaginary,  of  magnetism  and  electricity. 
But  why  have  recourse  to  invisible  agents  ?  Quod  petis,  hie 
est.  It  is  merely  the  ambient  air."  Thus  Herschel's  heat- 
ing effects  in  the  infra-red  were  attributed  to  currents  of  air 
from  the  visible  part  of  the  spectrum.  However,  Leslie  found 
no  followers,  after  Sir  Humphry  Davy  had  shown  that  in  a 
partial  vacuum  the  radiation  was  three  times  greater  than 

1  Phil.  Trans.,  1800,  p.  255. 


172  A   HISTORY   OF   PHYSICS 

in  air  at  ordinary  pressure,  and  after  JoJiann  Wilhelm  Rittet 
(1776-1810)  and  WoUaston  had  discovered  dark  chemical  rays 
in  the  ultra-violet.^  In  1811  a  young  Frenchman,  De  la  Eoche, 
showed  that,  of  two  successive  screens  of  the  same  kind,  the 
second  absorbs  heat  in  a  less  ratio  than  the  first,  and  he  con- 
cluded that  radiant  heat  is  of  different  kinds. ^  W.  Herschel 
had  previously  shown  that  "  radiant  heat  is  of  different 
refrangibilit3^'' 

But  no  marked  progress  was  made  in  the  knowledge  of 
radiant  heat  until  Macedonio  Melloni  (1798-1854)  ^  began  his 
researches.  From  early  boyhood  he  displayed  great  love  for 
science.  He  "was  born  a  physicist,"  and  began  to  teach 
physics  as  soon  as  he  left  the  school-bench.  For  seven  years 
he  taught  at  the  University  of  Parma.  Political  troubles  ban- 
ished him  from  Italy.  In  France  he  found  in  Arago  a  good 
friend.  Melloni  accepted  a  professorship  in  the  Department 
of  the  Jura,  but  in  1837  he  was  permitted  to  return  to  his 
native  country,  where,  in  1839,  he  was  appointed  Director  of 
the  Cabinet  of  Arts  and  Trades  in  Naples.'^  In  1850  Melloni 
published  a  great  work.  La  Thermoclirose,  ou  la  coloration 
calorifique,  in  which  he  embodied  his  researches  on  radiant 
energy.  In  the  preface  he  gives  the  story  of  his  early  passion 
for  nature.     The  passage  is,  in  part,  as  follows  : 

"  I  was  born  at  Parma,  and  when  I  got  a  holiday  used  to  go 
into  the  country  the  night  before,  and  go  to  bed  early,  so  as 
to  get  up  before  the  dawn.  Then  I  used  to  ^teal  silently  out 
of  the  house,  and  run,  with  bounding  heart,  till  I  got  to  the 

1  ROSENBERGER,  III,,  p.  67. 

2  S.  P.  Langley,  Address  before  A.A.A.S.,  1888,  p.  14. 

3  We  have  taken  these  dates  from  Rosenberger.  Marie  and  Larousse 
give  1801-1853. 

4  J.  Lovering's  biographic  sketch  in  Proc.  of  the  Am.  Acad,  of  Arts 
and  Sci.,  Vol.  III.,  1857,  p.  164. 


LIGHT  173 

top  of  a  little  hill,  where  I  used  to  set  myself  so  as  to  look 
toward  the  east."  There,  he  tells  us,  he  used  to  wait  the 
rising  sun  and  enjoy  the  glorious  spectacle.  "But  nothing," 
he  continues,  "so  rapt  my  imagination  as  the  bond,  so  inti- 
mate, which  unites  the  phenomena  of  life  to  the  brilliant  star 
of  day,"  whose  beams  are  accompanied  by  mysterious  heat.-^ 

To  insure  progress  in  the  study  of  radiant  heat  it  was  neces- 
sary that  the  thermometer  used  by  Herschel  be  superseded 
by  a  more  delicate  instrument.  Such  a  one  was  the  thermo- 
multiplier,^  or  thermopile,  invented  by  Leopoldo  Nobili  (1784- 
1835),  professor  in  Florence,  and  perfected  by  him  and  Melloni. 
One  of  the  results,  recognized  more  or  less  clearly  by  W.  Her- 
schel, De  la  Eoche,  and  others,  was  emphasized  by  Melloni ; 
viz.  that  radiant  heat  is  of  different  kinds,  that  there  is 
variety  among  heat  rays  just  as  there  is  variety  among  the 
visible  rays.  The  colour  of  heat,  as  the  phenomenon  is  meta- 
phorically called  by  Melloni,  is  not  perceived  by  the  eye,  but 
can  be  detected,  as  can  colours  of  light,  by  prismatic  dispersion 
or  by  experiments  in  which  some  colour  varieties  are  absorbed 
more  than  others.  Melloni  invented  the  word  thermochrdse, 
signifying  "  heat-colour."  He  arrived  at  a  close  realization  of 
the  identity  of  radiant  heat  and  light.  In  1843  he  said, 
"Light  is  merely  a  series  of  calorific  indications  sensible  to 
the  organs  of  sight,  or  vice  versa,  the  radiations  of  obscure 
heat  are  veritable  invisible  radiations  of  light."  ^  But  if  it  is 
true  that  where  light  is  there  must  be  radiant  heat,  then  lunar 
rays  must  exhibit  heat-effects.  He  tried  the  experiment,  failed 
at  first,  but  succeeded  afterwards.  On  Mount  Vesuvius,  in 
1846,  by  the  employment  of  a  polyzonal  lens,  one  metre  in 

^  Langlet,  op.  cit.,  p.  16. 

2  Fogg.  Annal,  Vol.  20,  1830,  p.  245 ;  Vol.  24,  1831,  p.  640. 

2  Translated  by  Langley,  op.  cit. ,  p.  18. 


174  A   HISTORY   OF  PHYSICS 

diameter,  together  with,  a  thermopile  and  galvanometer,  he 
succeeded  in  getting  feeble  indications  of  heat  from  kinar 
rays.  Melloni  made  numerous  experiments  on  the  absorption 
of  radiant  heat  by  solids  and  liquids.  He  coined  the  word 
diathermancy,  which  has  the  same  significance  for  radiant  heat 
that  the  word  transparency  has  for  visible  light.  In  his  ex- 
periments, the  radiation  from  a  lamp,  or  other  source,  was 
allowed  to  pass  through  the  air  to  the  thermopile ;  the  deflec- 
tion of  the  galvanometer  was  then  noted.  Next,  the  substance 
whose  diathermancy  was  to  be  tested  (water,  rock-salt,  glass, 
or  ice)  was  placed  in  the  path  of  the  rays  directed  upon  the 
pile,  and  the  consequent  deflection  noted.  Melloni's  tests 
seemed  to  show  that  rock-salt  was  perfectly  transparent  to 
all  kinds  of  calorific  rays — a  conclusion  now  known  to  require 
some  qualification.  Ice  and  glass  absorb  most  of  these  rays. 
Melloni  demonstrated  clearly  that  different  solids  and  liquids 
possess, different  transmissive  powers  and  that  (except  in  rock- 
salt)  the  diathermancy  varies  with  the  source  of  the  heat. 
Glass  transmits  39%  of  the  radiation  from  a  Locatelli  lamp, 
but  only  6%  of  that  from  copper  at  400°  C. 

While  Melloni  measured  the  diathermancy  of  different 
thicknesses  of  solids  and  liquids,  John  Tyndall  (1820-1893) 
effected  the  same  for  gases  and  vapours.  Tyndall  was  born 
near  Carlow  in  Ireland.  When  about  twenty-one  years  old 
he  went  to  England  and  attached  himself  to  a  Manchester 
firm  of  railway  engineers.  In  1847  he  accepted  a  position  as 
teacher  of  mathematics  and  surveying  in  the  newly  established 
G-reenwood  College,  where  science  was  to  be  taught  experi- 
mentally. About  one  year  later  he  went  to  the  University 
of  Marburg  to  study  mathematics,  physics,  and  chemistry. 
The  last  study  was  taken  under  Bunsen.  A  strong  influence 
toward  physics  was  exerted  upon  him  by  Karl  Hermann 
Knoblauch,  born  1820,  who  had  verified  and  extended  Mel 


LIGHT  llh 

loni's  work  on  radiant  energy.  After  graduating  in  1850j 
Tyndall  went  to  Berlin  and  worked  one  year  in  Magnus's 
laboratory  on  diamagnetism  and  magne-crystallic  action. 
After  his  return  to  England  lie  delivered,  in  1853,  a  lecture 
at  the  E-oyal  Institution  which  '^  took  his  audience  by  storm."  ^ 
He  was  elected  professor  of  natui-al  philosophy  in  the  E-oyal 
Institution,  which  had  become  famous  through  the  labours  of 
Thomas  Young,  Sir  Humphry  Davy,  and  Faraday.  It  was 
in  the  laboratory  of  that  place  that  Tyndall's  subsequent 
researches  were  made,  except  his  observations  of  natural  phe- 
nomena in  the  Swiss  Alps  during  his  vacations.  His  most 
important  original  work  was  in  the  domain  of  heat.  He  pos- 
sessed extraordinary  powers  of  popularizing  difficult  subjects. 
Perhaps  his  greatest  services  to  science  are  through  his  books, 
Heat  a  Mode  of  Motion,  Six  Lectures  on  Light  (delivered  in 
America  in  1872-1873),  Forms  of  Water,  etc.,  which  are  models 
of  popular  exposition. 

Melloni  had  concluded  from  experiments  with  his  thermo- 
electric apparatus  that,  for  a  distance  of  18  or  20  feet,  the 
absorption  of  radiant  heat  by  atmospheric  air  is  perfectly  in- 
sensible. Tyndall,  with  more  delicate  appliances,  verified  this 
conclusion :  dry  air  is  a  practical  vacuum,  as  regards  the  rays 
of  heat.  In  general,  the  elementary  gases  absorb  scarcely 
perceptible  amounts  of  radiant  heat.  But  Tyndall  found  it 
different  with  compound  gases  ;  they  absorb  portions  varying 
directly  with  the  complexity  of  their  molecules.  Thus  the 
vapour  of  ether,  having  fifteen  atoms  in  one  molecule,  absorbed 

IE.  Frankland,  "John  Tyndall,"  in  Froc.  Boyal  Soc.  of  London, 
Vol.  55,  1894,  p.  xviii.  For  Tyndall's  celebrated  "Prayer-test"  see 
Contemporary  Beview,  Vol.  20,  1872,  pp.  205-210.  The  same  volume 
contains  replies  by  James  M''Cosh  and  others.  Tyndall's  "Belfast 
Address,"  which,  at  the  time,  brought  upon  him  the  charge  of  "infi- 
delity," is  given  in  Beport  of  British  Association,  1874. 


176  A   HISTORY    OF   PHYSICS 

for  equal  volumes  at  maximum  density,  100  times  the  quantity 
of  radiant  heat  intercepted  by  the  vapour  of  carbon  disulphide, 
containing  only  three  atoms.  Tyndall  found  that  the  radiating 
powers  follow  precisely  the  same  order  as  the  powers  of 
absorption.  Thus,  oxygen,  hydrogen,  and  nitrogen  do  not 
radiate  heat,  while  ammonia  will  show  decided  effects.  The 
same  subject  was  investigated  by  H.  G.  Magnus  of  Berlin,  and 
the  agreement  between  the  two  investigators  was  very  close, 
except  in  case  of  aqueous  vapour.  Magnus  found  that  it  had 
little  or  no  action;  Tyndall  found  it  to  be  considerable  for 
heat  rays  of  low  refrangibility.  The  question  is  an  important 
one  in  meteorology.  The  controversy  lasted  many  years.^ 
But  in  1881  Tyndall  published  a  paper  ^  which  finally  proved 
that  he  was  right.  At  that  time  Alexander  Graham  Bell  had 
obtained  musical  sounds  through  the  action  of  an  intermittent 
beam  of  light  falling  upon  solid  bodies  enclosed  in  a  glass 
flask.  The  ear  was  placed  in  communication  with  the  interior 
of  the  flask  by  means  of  a  hearing  tube.  When  a  beam  of 
light  fell  upon  the  substance  in  the  tube,  it  expanded  and  a 
pulse  of  air  was  expelled.  When  the  light  was  cut  off,  the 
opposite  effect  took  place.  Thus,  sound  was  produced.  Bell 
showed  some  of  these  experiments  to  Tyndall  in  the  laboratory 
of  the  Boyal  Institution,  whereupon  Tyndall  made  experi- 
ments on  flasks  filled  with  different  gases.^  He  says  that 
when  a  flask  containing  moist  air  was  placed  in  the  inter- 
mittent beam,  "  I  heard  a  powerful  musical  sound  produced  by 

1  For  historic  remarks  on  this  poi.'it,  consult  Tyndall,  Contributions 
to  Molecular  Physics  in  the  Domain  of  Badiant  Heat.,  London,  1872, 
pp.  59-64. 

2  "  Action  of  an  Intermittent  Beam  ^f  Radiant  Heat  upon  Gaseous 
Matter,"  Proc.  Boy.  Soc,  Vol.  31,  1881,  p.  307;  Nature,  Vol.  25,  1882, 
pp.  232-234. 

3  A.  G.  Bell,  Upon  the  Production  of"  Sound  by  Badiant  Energy^ 
Washington,  1881,  p.  19. 


LIGHT  177 

the  aqueous  vapor.  I  placed  the  flask  in  cold  water  until  its 
temperature  was  reduced  from  about  90°  to  10°  C,  fully  ex- 
pecting the  same  sound  would  vanish  at  this  temperature ;  but 
.  .  .  the  sound  was  distinct  and  loud.  Three  empty  flasks 
filled  with  ordinary  air  were  placed  in  a  freezing  mixture. 
On  being  rapidly  transferred  to  the  intermittent  beam,  sounds 
much  louder  than  those  obtained  from  dry  air  were  produced." 
Thus  the  aqueous  vapour  showed  absorption,  and  the  contro- 
versy was  finally  ended. 

Leslie,  Melloni,  and  Tyndall  pointed  out  an  error  of  wide 
prevalence  regarding  -the  influence  of  colour  on  absorption. 
Benjamin  Pranklin  had  placed  cloths  of  various  colours  upon 
snow  and  allowed  the  sun  to  shine  upon  them.  They  absorbed 
solar  rays  to  different  degrees  and  sank  to  different  depths  in 
the  snow.  From  this  Franklin  concluded  that  dark  colours 
were  the  best  absorbers,  and  light  colours  the  worst.  But  this 
generalization  requires  qualification.  Did  the  radiation  from 
the  sun  or  other  luminous  body  consist  exclusively  of  visible 
rays,  then  the  problem  would  be  simpler,  but  the  invisible  rays" 
often  produce  effects  exactly  opposite  to  what  Franklin's 
theory  would  lead  us  to  expect.  Tyndall  coated  the  bulb  of  a 
delicate  mercury  thermometer  with  alum  (a  white  powder),  and 
the  bulb  of  a  second  thermometer  with  iodine  (a  dark  powder). 
On  exposing  the  bulbs  at  the  same  distance  to  the  radiation 
from  a  gas  flame,  the  alum-covered  thermometer  rose  nearly 
twice  as  high  as  the  other ;  alum  was  a  better  absorber  than 
iodine.  Tyndall  says  that  "radiation  from  the  clothes  which 
cover  the  human  body  is  not  at  all,  to  the  extent  sometimes 
supposed,  dependent  on  their  color.  The  color  of  animal's  fur 
is  equally  incompetent  to  influence  radiation.  These  are  the 
conclusions  arrived  at  by  Leslie  and  Melloni  fo7' obscure  heat.'' 

ITxNDALL,  Meat  a  Mode  of  Motion,  New  York,  1897,  p.  299. 


178  A   HISTORY   OF   PHYSICS 

Important  contributions  to  our  knowledge  of  radiant  energy 
were  made  by  Samuel  Pierpont  Langley  (born  1834).     In  1867 
he  became  professor  of  astronomy  in  th.e  Western  University 
of  Pennsylvania,  with  charge  of  the  observatory  at  Allegheny 
City.     Since  1887  he  has  been  connected  with  the  Smithsonian 
Institution  as  secretary.     To  make  marked  progress  in  the 
study   of   radiation  it   seemed  necessary   to   invent    a   more 
delicate    instrument    than    the    thermopile    of    Melloni    and 
Tyndall.     Langley's  new  device  was  the  bolometer,  first  de- 
scribed in  1881.^    A  very  fine  strip  of  platinum  (at  first  iron 
was  used)  serves  as  a  conducting  wire  in  a  circuit.     If  radia- 
tion falls  upon  it,  its  temperature  is  raised  and  its  electric 
resistance  increased.      A   delicate   galvanometer   records   the 
resulting  disturbance  in  the  electric  current.     The  bolometer 
has  been  made  to  indicate  a  change  of  temperature  of  .0000001 
of  a  degree  centigrade.     Some  of  its  first  results  were  to  show 
that  the  maximum  heat  in  the  solar  spectrum  was   in  the 
orange,  not  in  the  infra-red,  as  claimed  by  W.  Herschel  and 
others.      The  earlier  observers  had  used  the  prismatic  spec- 
trum, which  is  subject  to  two  important  errors :  (1)  the  prism 
absorbs  part  of  the  radiation,  exercising  so-called  "selective 
absorption  " ;  (2)  the  prism  concentrates  the  rays  in  the  lower 
part  of  the  spectrum  as  compared  with  the  upper,  thus  falsify- 
ing the  true  distribution  of  heat.     These  errors  are  avoided  by 
the  use  of  a  grating,  which  yields  a  "  normal  spectrum."     Tor 
many  years  the  belief  (unsupported  by  experiment)  was  preva- 
lent that  our  atmosphere  acted  exactly  the  part  of  glass  in  a 
hot-bed,  and  that  it  kept  the  planet  warm  by  absorbing  the 
infra-red  rays  radiated  by  the  earth.     Langley  proved  experi- 

^Am.  Jour.  Sci.  (3)  Vol.  21,  pp.  187-198.  The  latest  form  of  it  is 
described  in  the  same  journal,  (4)  Vol.  5,  1898,  pp.  241-245.  For  a 
biographical  sketch  of  Langley,  see  Fopular  Science  Monthly^  Vol.  27, 
1885,  pp.  401-409. 


LIGHT  179 

mentally  tliat  this  was  not  true.  The  infra-red  rays  pass 
through  with  comparative  ease.  His  experiments  at  Allegheny 
City,  continued  in  1881  on  the  crest  of  Mount  Whitney  in  the 
Sierra  Nevadas/  showed  that  the  atmosphere  acts  "  with  selec^ 
tive  absorption  to  an  unanticipated  degree,  keeping  back  an 
immense  proportion  of  the  blue  and  green."  The  atmosphere 
not  only  keeps  back  a  part  of  the  solar  radiation,  but  totally 
changes  its  composition  in  doing  so.  By  taking  out  more  of 
the  blue  and  green,  the  residue  coming  down  to  us  produces 
the  sensation  of  what  is  familiarly  known  as  "  white  "  light, 
so  that  "  white  "  is  not  the  sum  of  all  the  radiation  from  the 
sun.  Could  we  rise  above  the  earth's  atmosphere,  then  the 
sun  would  appear  to  us  greenish  blue.  The  pure  original  sun- 
light is  no  more  like  the  radiation  falling  upon  the  earth's 
surface  than  the  electric  light  is  like  that  which  reaches  the 
eye  through  reddish  glasses. 

Assisted  by  F.  W.  Very,  Langley  experimented  on  the  tem- 
perature of  the  moon.^  The  bolometer  "gave  indications  of 
two  maxima  in  the  heat-curve,  the  first  corresponding  to  the 
heat  from  the  solar  reflected  rays,  the  second  (indefinitely 
lower  down  in  the  spectrum)  corresponding  to  a  greater 
amount  of  radiant  heat  emitted  from  a  source  at  a  far  lower 
temperature,"  viz.  from  the  surface  of  the  moon  itself.  The 
mean  temperature  of  the  sunlit  lunar  soil  "  is  most  probably 
not  greatly  above  zero  centigrade."  The  determination  is 
founded  on  the  fact,  experimentally  established  by  Langley, 
"that  the  position  of  the  maximum  in  a  curve,  representing 
invisible  radiant  heat,  furnishes  a  reliable  criterion  as  to  the 
temperature  of  the  radiating  (solid)  body." 

^Am.  Jour.  Sci.  (3),  Vol.  25,  1883,  pp.  169-196. 

2  Am.  Jour.  Sci.,  Vol.  38,  1889,  pp.  421-440.  This  is  only  in  abstract. 
The  full  memoir  appeared  in  Memoirs  of  the  National  Acad,  of  Sciences^ 
Vol.  IV. 


180  A   HISTORY   OF  PHYSICS 

By  the  study  of  the  radiation  of  the  fire-fly,  Langley  and 
Very  showed^  "that  it  is  possible  to  produce  light  without 
heat,  other  than  that  in  the  light  itself  5  that  this  is  actually 
effected  now  by  nature's  processes'';  "that  nature  produces 
this  cheapest  light  at  about  one  four-hundredth  part  of  the 
cost  of  the  energy  which  is  expended  in  the  candle-flame,  and 
at  but  an  insignificant  fraction  of  the  cost  of  the  electric 
light." 

Langley  demonstrated  that  the  visual  effect  produced  by 
any  given  constant  amount  of  energy  varies  enormously  ac- 
cording to  the  colour  of  the  light  in  question.  For  the  same 
colour,  it  varies  with  eyes  of  different  individuals.  The  sensa- 
tion of  crimson  light  ordinarily  requires  that  the  energy  of  the 
waves  arrested  by  the  retina,  during  the  act  of  perception,  be 
about  .001  of  an  erg,  while  the  sensation  of  green  can  be  pro- 
duced by  .000,000,01  of  an  erg.  In  other  words,  about  100,000 
times  the  energy  is  demanded  to  make  us  see  red  that  is 
needed  to  make  us  see  green.^ 

Langley  explored  widely  the  infra-red  region  of  the  solar 
spectrum.  J.  W.  Draper  had,  in  his  photograph  of  1842, 
observed  three  wide  bands  in  this  region.  The  same  were 
noticed  by  Foucault  and  Fizeau  in  1846.  Captain  W.  de  W. 
Ahney  in  1880  mapped  by  photography  the  infra-red  prismatic 
spectrum  as  far  as  1.075'^.  Langley  got  heating  effects  of 
more  than  twice  that  wave-length;  his  delicate  filament  of 
platinum  groping  its  way  down  to  nearly  3'^,  that  is,  nearly  to 
rays  of  .003  mm.  wave-length.  At  this  place  solar  heat  seems 
to  be  abruptly  cut  off.  The  visible  part  of  the  solar  spectrum 
extends  from  about  the  line  H=  .39'"  to  ^  =  .76'" ;  the  invisi- 
ble spectrum,  as  explored  by  Langley,  reaches  from  .76'*  to 


1  Am.  Journ.  Sci.  (3),  Vol.  40,  1890,  pp.  97-113. 

2  Langley,  Phil.  Mag.,  Vol.  27,  1889,  p.  23. 


LIGHT  181 

nearly  3'*.  Langley  studied  also  invisible  infra-red  radiations 
from  terrestrial  sources,  and  has  learned  with  certainty  of 
wave-lengths  greater  than  .005  mm.,  and  has  grounds  for 
estimating  that  he  has  recognized  radiations  whose  wave- 
length exceeds  .03  mm.,  so  that,  while  he  has  directly  meas- 
ured nearly  eight  times  the  wave-length  known  to  Newton,  he 
has  probable  indications  of  wave-lengths  much  greater.^  In 
place  of  the  bolometer,  Rubens  and  E.  F.  Nichols  have  used  a 
modified  form  of  Crookes's  radiometer.  They  have  isolated 
and  identified  rays  from  hot  zirconia  of  .05  mm.  wave-length.^ 
These  are  about  j^-g-  the  length  to  the  shortest  Hertzian 
waves.  Thus,  homogeneous  radiation  of  nearly  every  wave- 
length from  Hertzian  waves,  several  kilometers  long,  down  to 
ultra-violet  rays,  less  than  .0002  mm.,  is  definitely  known. 

The  ultra-violet  rays  in  the  solar  spectrum,  the  existence  of 
which  was  discovered  by  Hitter  and  WoUaston,  were  studied  by 
Biot,  who  found  that  the  absorptive  power  of  glass,  rock-salt, 
and  quartz  for  these  rays  is  independent  of  their  absorptive 
power  for  visible  rays.  A.  C.  Becquerel  showed  that  quartz 
is  especially  transparent  to  these  rays ;  even  a  dark  piece 
will  let  more  through  than  a  clear  pane  of  glass.  The  ultra- 
violet region  has  been  studied  both  for  solar  and  artificial 
radiation.  Franz  Exner  and  E.  Haschek,  using  a  Rowland 
grating,  in  1896  studied  the  spark  spectra  of  eleven  metals 
by  photography,  and  took  measurements  on  more  than  19,000 
ultra-violet  lines.^ 

Curious  observations  have  been  made  on  "anomalous  dis- 
persion."    The  existence  of   this  phenomenon  was  first  dis- 

1  Am.  Jotir.  Set  (3),  Vol.  32,  1886,  p.  24.  For  still  more  recent  results, 
see  Nature,  Vol.  51,  pp.  12-16,  1894. 

2  Physical  Beview,  Vol.  IV.,  1897,  pp.  314-323. 

^  Sitzungsherichte  d.  K.  Akad.  d.  W.  Wien,  Vol.  105,  pp.  389-486, 
603-574,  707-740;  Astrophys.  Jonr.,  Vol.  5,  1897,  p.  290. 


182  A   HISTORY   OF   PHYSICS 

covered  by  Fox  Talbot  about  1840.  The  term  was  invented  in 
1862  by  Le  Roux,^  who  noticed  that  the  vapour  of  iodine 
absorbs  the  middle  part  of  the  visible  spectrum ;  and  that,  as 
compared  with  other  bodies,  it  refracts  the  blue  to  a  less 
degree  than  the  red.  In  1870  C.  Christiayisen  saw  that  a 
hollow  glass  prism  filled  with  a  solution  of  fuchsine  gave  the 
order  of  the  colours,  violet,  red,  yellow,  instead  of  red,  yellow, 
violet.  August  Kundt  (1839-1894),  professor  at  Wurzburg, 
after  1888  professor  in  Berlin,  has  described  a  similar  behaviour 
in  cyan,  mauve  anilin,  anilin  blue,  and  other  substances  ^  whose 
colour  by  reflection  is  different  from  their  colour  by  transmission. 
His  observations  were  not  confined  to  substances  in  the  liquid 
state.  In  1880  he  accidentally  discovered  anomalous  disper- 
sion in  the  vapour  of  sodium.  In  the  dispersion,  caused  by 
very  thin  films  of  certain  metals,  Kundt  noticed  a  strange 
fact.  In  gold,  silver,  and  copper  the  ray  was  bent  away  from 
the  normal,  on  passing  from  air  into  the  film;  that  is,  the 
index  of  refraction  turned  out  to  be  less  than  unity.  For 
gold  and  copper  the  red  ray  was  bent  further  from  the  normal 
than  the  blue  ray.  For  platinum,  iron,  nickel,  bismuth,  the 
index  was  greater  than  unity,  and  in  each  case  greater  for  the 
red  light  than  the  blue,  showing  that  the  red  was  deviated 
further  toward  the  perpendicular  than  the  blue.  The  prepara- 
tion of  the  metallic  prisms  of  very  small  angle  and  of  suffi- 
cient transparency  for  these  experiments  was  effected  by 
electrolytic  deposition  upon  platinized  glass.  This  work  oc- 
cupied two  years,  and  the  small  number  of  usable  prisms  was 
chosen  out  of  more  than  2000  made.  The  velocity  of  light 
in  these  metals  stands  in  close  relationship  to  their  power  of 
conducting  electricity  and  heat;    the  greatest  velocity  being 

1  Compt.  Bend.,  Vol.  55,  p.  126. 

^Pogg.  Ann.,   Vol.    142,  p.  163;  Vol.  143,  pp.   149,   259;  Vol.  144, 
p  '*'28 ;  Vol.  145,  pp.  67,  164. 


LIGHT  18b 

through  the  best  conductors.^  The  phenomena  of  anomalous 
dispersion  play  an  important  role  in  modern  theories  of  dis- 
persion, advanced  by  Ketteler,  Helmholtz,  and  others. 

The  nomenclature  of  the  subject  of  radiant  energy  is  in 
need  of  revision.  The  expression  "  radiant  heat "  is  still  much 
used ;  but  the  term  is  self-contradictory,  if  by  heat  we  mean 
a  form  of  energy  due  to  molecular  motion  in  ponderable 
matter.  Where  there  are  no  molecules  there  can  be  no  heat. 
The  phenomena  of  "radiant  heat"  do  not  belong  to  the 
science  of  heat  at  all,  unless  we  resort  to  the  objectionable 
course  of  attaching  a  double  meaning  to  the  word  "  heat "  by 
allowing  it  to  designate  a  form  of  energy  due  to  ethereal 
waves  as  well  as  that  due  to  molecular  agitation.^  The  terms 
"  diathermanous  "  and  "  athermanous  "  are  ill  chosen,  because 
they  etymologically  refer  to  thermic  or  heat  phenomena,  when 
really  we  are  dealing  with  ether-waves.^ 

The  problem  of  photography  in  natural  colours  is  as  old  as 
photography  itself.  The  first  efforts  at  solution  were  by  the 
chemical  method.  The  trials  which  are  best  known  are  those 
made  by  Edmond  Becquerel,  who  succeeded  in  obtaining  upon 
a  silver  plate  covered  with  a  film  of  violet  subchloride  of 
silver  the  impression  of  all  the  colours  of  the  solar  spectrum. 
But  they  vanished  as  soon  as  they  were  exposed  to  the  light.* 

In  the  second  method  of  colour  photography  three  separate 
colourless  negatives  are  taken  of  an  object  by  light  passing 
through  three  differently  coloured  screens.  From  these,  three 
colourless  positives  are  made.  Then  each  positive  is  dyed  with 
the  colour  corresponding  to  the  light  used  in  obtaining  its 

1  KuNDT,  Phil  Mag.  (5),  Vol.  26,  1888,  p.  2. 

2  The  word  "  heat  "  is  so  defined  in  the  Standard  Dictionary. 

3  The  nomenclature  of  radiant  energy  is  discussed  in  Nature,  Vol.  49, 
1893,  pp.  100,  149,  389. 

*  L.  Weiller  in  Pop.  Sci.  Monthly,  Vol.  45,  1894,  p.  539- 


184  A   HISTORY   OF   PHYSICS 

negative.  On  superimposing  the  coloured  positives  and  viewing 
them,  by  transmitted  light;  the  object  photographed  is  seen  in 
its  natural  colours.  This  process  was  invented  in  France  by 
Charles  Cros  and  at  the  same  time  (1869)  by  Ducos  du  Hauron. 
The  Germans  claim  the  priority  of  the  idea  for  Baron  Bon- 
stetten.     The  process  has  been  improved  by  J.  Joly} 

The  third  method,  due  to  interference  of  light,  was  pub- 
lished by  O.  Lippmann  of  Paris.^  A  transparent  photographic 
film  is  placed  in  contact  with  a  layer  of  mercury.  The  light 
reflected  from  the  mercury  interferes  with  the  incident  light 
so  as  to  form  standing  waves  in  the  film.  In  this  way  the 
film  is  divided  into  a  number  of  thin,  equidistant  strata, 
parallel  to  the  surface  of  the  glass.  The  distance  between 
these  layers  is  half  the  wave-length  of  the  incident  light. 
They  act  as  reflecting  surfaces,  and  appear  coloured  when 
viewed  at  the  proper  angle.  Thus,  if  the  strata  at  any  point 
are  formed  by  violet  light,  they  will  reflect  only  violet  light. 
It  is  interesting  to  notice  that  Lippmann  was  led  to  these 
experiments  through  an  effort  to  transport  into  the  domain  of 
light  the  acoustic  property  of  an  organ-pipe,  according  to 
which  the  fundamental  pitch  which  it  gives  forth  depends 
only  upon  its  length. 

The  unique  idea  of  adopting  the  wave-length  of  some  partic- 
ular ray  of  light  as  a  "  standard  of  length  "  was  first  advanced 
in  1829  by  the  Frenchman  Jacques  Babinet  (1794-1872). ^  The 
wave-lengths  of  light  were  assumed  to  be  of  constant  value. 
The  first  attempt  to  carry  out  this  plan  was  made  by  C.  JS. 
Peirce,  in  conjunction  with  Rutlierfurd.^  The  scheme  ap- 
proached more  nearly  to  a  practical  realization  in  the  hands 
of  A.  A.  Michelson  and  Edward  W.  Morley,  who  in  1887  sug- 

1  Nature,  Vol.  63,  1896,  p.  617.  2  Nature,  Vol.  53,  pp.  617,  618. 

«  RosENBERGER,  III.,  p.  193.  *  Natwe,  Vol.  20,  1879,  p.  99. 


LIGHT  185 

gested  the  wave-length,  of  sodium  light  as  the  standard  and 
explained  their  inferential  comparator  for  determining  the 
length  of  the  metre  in  terms  of  the  wave-length.^  Later  a 
green  mercury  ray  was  tried  in  place  of  the  sodium  light.^  In 
1892  Michelson,  by  invitation,  took  his  apparatus  from  Clark 
University  to  Paris,  for  the  purpose  of  instituting  a  com- 
parison of  the  length  of  the  new  international  metre,  with 
the  wave-lengths  of  certain  cadmium  lines  which  were  found 
to  be  preferable  to  others  on  account  of  their  great  homo- 
geneity. This  delicate  undertaking  was  carried  out  in  the 
Pavilion  de  Breteuil.^  Thus  the  fundamental  unit  of  the 
metric  system  was  compared  "with  a  natural  unit  with 
the  same  degree  of  approximation  as  that  which  obtains  in 
the  comparison  of  two  standard  metres.  This  natural  unit 
depends  only  on  the  properties  of  the  vibrating  atoms  and 
of  the  universal  ether ;  it  is  thus,  in  all  probability,  one  of  the 
most  constant  dimensions  in  all  nature." 

The  first  step  toward  modern  theories  of  vision  was  taken 
by  Thomas  Young  in  1801  in  his  article  on  Light  and  Colours. 
He  there  makes  the  hypothetical  statement  that  each  part  of 
the  retina  has  three  particles,  sensitive  respectively  to  the 
colours  red,  yellow,  and  hlue.'^  Thereupon  Wollaston  made  his 
celebrated  observation  of  dark  lines  in  the  solar  spectrum, 
which  he  supposed  to  be  the  dividing  lines  of  the  pure  simple 
colours.  This  misconception  led  Young  to  choose  red,  green, 
and  violet  as  colours  primarily  perceived,  in  place  of  red,  yellow, 
and  hlue.^    Later,  Young  found,  by  the  rotation  of  coloured 

1  Am.  Jour.  Set,  Vol.  34,  1887,  pp.  427-430. 

2  Am.  Jour.  Set.,  Vol.  38,  1889,  p.  181. 

3  Compt.  Bend.,  Vol.  116,  1893,  pp.  790-794;  Astronomy  and  Astro- 
Physics,  Vol.  XII.,  1893,  pp.  656-560. 

*  Miscellaneous  Works,  Vol.  I,,  p.  147. 
5  Ibidem,  I.,  p.  177. 


186  A  HISTORY   OF  PHYSICS 

disks,  that  a  mixture  of  red,  green,  and  violet  produces  gray.^ 
Young's  theory  was  elaborated  by  two  of  the  greatest  physi- 
cists of  the  century,  —  Helmholtz  ^  and  Maxwell.  It  is  mainly 
owing  to  their  labours  that  it  received  the  careful  consideration 
of  physicists.  The  main  support  for  the  selection  of  red, 
green,  and  violet  as  the  fundamental  triad  has  been  sought  in 
the  phenomenon  of  colour-blindness.  Blindness  to  red  is  the 
most  common ;  blindness  to  green  is  not  uncommon ;  to  violet, 
has  been  known  to  exist.  But  this  argument  is  not  so  con- 
clusive as  might  be  desired,  for  the  reason  that  a  deficiency  in 
colour  sense  for  red  is  at  times  accompanied  by  a  deficiency 
for  yellow,  green,  and  blue.  The  so-called  "  Young-Helmholtz 
theory''  has  been  extended  by  Ogden  JSf.  Mood,  of  Columbia 
College,  and  W.  de  W.  Ahney.  But  it  has  not  met  with  univer- 
sal acceptation.  A  rival  theory  was  advanced  by  E.  Hering  ^ 
of  Vienna,  who  accepts  three  elementary  sensations,  viz.  black 
and  white,  red  and  green,  blue  and  yellow.  Since  1887  the 
physiological  psychologists  have  begun  investigating  the  sub- 
ject, and  half  a  dozen  new  theories  have  been  advanced. 
Among  physicists  the  Young-Helmholtz  theory,  on  account  of 
its  simplicity,  is  meeting  with  greater  favour  than  any  other.* 
A  strong  committee  of  English  physicists,  a  few  years  ago, 
reported  on  colour-vision.  It  adopted  the  Young-Helmholtz 
theory,  but  pointed  out  that  it  fails  to  explain  some  curious 
cases  of  diseased  vision  in  which  the  sensation  of  colour  is  con- 
fined to  the  blue  end  of  the  spectrum.     On  the  other  hand, 

1  Natural  Philosophy,  1807.  See  also  A.  M.  Mayer  in  Phil.  Mag. 
(5),  Vol.  I.,  1876. 

2  See  Helmholtz,  Physiologische  Optik,  1856 ;  L.  Campbell  and 
W.  Garnett,  The  Life  of  James  Clerk  Maxwell,  London,  1882,  pp. 
4i6-483. 

3  Lehre  vom  Lichtsinne,  Vienna,  1878. 

*  Consult  W.  Le  Conte  Stevens  in  Science,  N.  S.,  Vo;.  7,  1898,  pp. 
513-520;  F.  P.  Whitman  in  Science,  N.  S.,  Vol.  8,  1898,  pp.  306-316. 


LIGHT         .  187 

Hering's  theory  was  also  declared  to  fail  to  account  for  some 
known  facts. ^ 

Once  there  was  a  wide-spread  conviction  that  the  human 
eye  was  an  optical  instrument  of  such  great  perfection  that 
none  formed  by  human  hands  could  rival  it.  Actual  examina- 
tion of  the  action  of  the  eye,  carried  on  mainly  by  Helmholtz, 
has  brought  about  a  change  in  these  views.  Says  he,  "Now 
it  is  not  too  much  to  say  that  if  an  optician  wanted  to  sell  me 
an  instrument  which  had  all  these  defects,  I  should  think  my- 
self quite  justified  in  blaming  his  carelessness  in  the  strongest 
terms,  and  giving  him  back  his  instrument."  ^  This  statement 
is  supported  by  passages  like  the  following:^  "A  refracting 
surface  which  is  imperfectly  elliptical,  an  ill-centred  tele- 
scope, does  not  give  a  single  illuminated  point  as  the  image 
of  a  star,  but  according  to  the  surface  and  arrangement  of  the 
refracting  media,  elliptic,  circular,  or  linear  images.  Now  the 
images  of  an  illuminated  point,  as  the  human  eye  brings  them 
to  focus,  are  even  more  inaccurate:  they  are  irregularly  radi- 
ated. The  reason  of  this  lies  in  the  construction  of  the  crys- 
talline lens,  the  fibres  of  which  are  arranged  around  six 
diverging  axes,  so  that  the  rays  which  we  see  around  stars 
and  other  distant  lights  are  images  of  the  radiated  structure 
of  our  lens ;  and  the  universality  of  this  optical  defect  is 
proved  by  any  figure  with  diverging  rays,  being  called  ^  star- 
shaped.'  It  is  from  the  same  cause  that  the  moon,  while  her 
crescent  is  still  narrow,  appears  to  many  persons  double  or 
threefold."  The  mechanism  by  which  the  eye  accommodates 
itself  to  viewing  objects  at  various  distances  was  a  great  riddle 
until  the  French  surgeon,  Louis  Joseph  Sanson,  first  observed 

1  See  Proc.  Boy.  8oc.  of  London,  Vol.  51,  1892,  pp.  281-395. 

2  H.  Helmholtz,  Popular  Lectures^  trans,  by  E.  Atkinson^  London, 
1873,  p.  219. 

3  Ibidem,  p.  218. 


188  A   HISTORY   OF   PHYSICS 

very  faint  reflection  of  light  through  the  pupil  from  the  twc 
surfaces  of  the  crystalline  lens.  Max  Lagenbeck  found  that 
this  reflection  altered  during  the  act  of  accommodation. 
Helmholtz  and  others,  by  these  alterations,  studied  the  changes 
of  the  lens,  and  arrived  at  the  conclusion  that  the  eye  adjusts 
itself  by  the  contraction  of  the  ciliary  muscle,  causing  the 
tension  of  the  lens  to  be  diminished  and  its  surfaces  (chiefly 
the  front  one)  to  become  more  convex  than  when  the  eye  is  at 
rest,  the  images  of  near  objects  being  thus  brought  to  a  focus 
on  the  retina.^ 

Helmholtz  irreverently  disclosed  the  fact  that  in  blue  eyes 
there  is  no  real  blue  colouring  matter  whatever ;  the  deepest 
blue  is  nothing  but  a  turbid  medium.  The  optic  action  is  the 
same  as  in  case  of  smoke  which  appears  blue  on  a  dark  back- 
ground, though  the  particles  themselves  are  not  blue ;  or  in 
case  of  the  sky,  which,  according  to  Newton,  Stokes  and  Eay- 
leigh,^  looks  blue  through  the  agency  of  extremely  fine  dust 
suspended  in  the  air.^  This  dust,  when  illuminated  by  sun- 
light, reflects  a  greater  proportion  of  the  shorter  waves  of 
bluish  light  and  transmits  a  greater  proportion  of  longer 
waves  of  reddish  light. 

Helmholtz  and  Maxwell  experimented  on  the  effects  pro- 
duced by  mixing  colours.  That  a  mixture  of  yellow  and  blue 
light  produces  gray  and  not  green  was  probably  first  pointed 
out  by  James  D.  Forbes} 

1  H.  Helmholtz,  Popular  Lectures^  trans,  by  E.  Atkinson^  London, 
1873,  p.  205. 

2  Phil.  Mag.,  Vol.  41,  pp.  107,  275. 

3  James  Dewar  attributes  the  blueness  of  the  sky  to  the  oxygen  in  the 
air  ;  liquelied  oxygen  is  blue. 

4  Campbell  and  Garnett,  The  Life  of  James  Cleric  Maxwell.,  London, 
1882,  p.  214.  Maxwell  "was  fond  of  insisting,  to  his  female  cousins, 
aunts,  etc.,  on  the  truth  that  blue  and  yellow  do  not  make  green.  I 
remember  his  explaining  to  me  [L.  Campbell]  the  difference  between 


HEAT  189 


HEAT 

Tlie  first  prominent  physicist  who  endeavoured  to  overthrow 
the  caloric  theory  of  heat  was  Benjamin  Thompson,  Count 
Rumford  (1753-1814).^  He  was  born  at  North  Woburn  in  a 
humble  New  England  home,  within  two  miles  of  the  native 
place  of  another  great  Benjamin,  —  Eranklin.  These  men 
never  met,  but  both  achieved  great  things  in  physical  investi- 
gation. In  Benjamin  Thompson  a  taste  for  research  displayed 
itself  early.  An  old  note-book  of  his  contains  the  entry  :  "  An 
account  of  what  work  I  have  done  towards  getting  an  Electrical 
Machine.  Two  or  three  days'  work  making  wheels.  One  half 
day's  work  making  pattern  for  small  Conductor.  Making 
pattern  for  Electrometer."  At  one  time  he  walked  eight 
miles  from  Woburn  to  Cambridge  to  attend  the  lectures  on 
natural  philosophy  of  Professor  John  Winthrop  at  Harvard 
College.^     At   the  age  of  nineteen  he  taught   school   in  the 

pigments  and  colors  "  (p.  198).  While  Maxwell  sometimes  mixed  colours 
by  rotation,  he  usually  used  a  "  colour-box"  which  he  perfected  in  1862. 
"  A  beam  of  sunlight  is  to  be  divided  into  colors  by  a  prism,  certain  colors 
selected  by  a  screen  with  slits.  These  gathered  by  a  lens,  and  restored 
to  the  form  of  a  beam  by  another  prism,  and  then  viewed  by  the  eye 
directly  "  (p.  334).  While  he  was  professor  at  King's  College,  he  resided 
at  8,  Palace  Gardens  Terrace,  Kensington,  "  where  he  carried  on  many  of 
his  experiments  in  a  large  garret  which  ran  the  whole  length  of  the  house. 
When  experimenting  at  the  window  with  the  color-box  (which  was 
painted  black,  and  nearly  eight  feet  long),  he  excited  the  wonder  of  his 
neighbors,  who  thought  him  mad  to  spend  so  many  hours  in  staring  into 
a  coffin"  (p.  318). 

1  G.  E.  Ellis,  3Iemoir  of  Sir  Benjamin  Thompson,  Count  Bumford, 
with  notices  of  his  daughter,  published  in  connection  with  an  edition  of 
Eumford's  complete  works,  by  the  American  Academy  of  Arts  and 
Sciences,  Boston. 

2  It  was  as  a  grateful  return  for  the  favours  he  had  thus  enjoyed  at  the 
college  that  Count  Rumford  later  gave  to  it  the  endowment  which 
founded  the  professorship  that  bears  his  name.     Ibidem,  p.  36. 


190  A  HISTORY   OF  PHYSICS 

district  at  Wilmington.  At  the  outbreak  of  the  War  of  the 
Eevolution  Thompson  seemed  to  favour  the  Tory  party ;  he 
was  viewed  with  suspicion  as  being  an  enemy  to  his  country  ; 
was  arrested  and  confined  in  Woburn.  Yet  no  positive  and 
direct  evidence  has  ever  been  found  of  any  unfriendly  act  done 
by  Thompson,  or  even  of  any  speech  of  such  a  character 
attributed  to  him.^  At  the  age  of  twenty-two,  Thompson  fled 
to  England,  leaving  behind  him  a  wife  and  daughter.  As  far 
as  known  he  never  even  wrote  to  his  wife.  In  this  prominent 
scientist,  "  the  life  of  the  intellect  appeared  to  have  interfered 
with  the  life  of  the  affections."  Eor'a  time  he  participated  in 
the  war  on  the  side  of  the  British. 

In  1777  he  began  his  career  as  an  experimental  scientist  by 
a  research  on  the  cohesive  strength  of  different  substances. 
The  following  year  he  was  admitted  as  a  fellow  of  the  Koyal 
Society.  Having  a  strong  predilection  for  a  military  life,  he 
left  England  in  1783  to  serve  with  the  Austrians  in  a  war  then 
meditated  against  the  Turks.  As  war  did  not  break  out,  he 
entered  into  the  service  of  the  Elector  of  Bavaria,  who,  in 
1790,  made  him  a  count.  He  established  houses  of  industry, 
schools  of  industry,  founded  a  military  academy  at  Munich, 
and  at  the  same  time  continued  his  physical  researches.^  On 
the  death  of  the  elector,  in  1799,  Bumford  went  to  London, 
where  he  founded  the  Royal  Institution  for  the  diffusion  of  a 
knowledge  of  applied  science.  It  is  pleasant  to  contemplate 
that,  while  the  Boyal  Institution  was  founded  by  an  American, 
the  Smithsonian  Institution,  in  Washington,  owes  its  origin  to 


1  Memoir  of  Sir  Benjamin  Thompson,  Count  Bumford,  p.  58. 

2  "His  labors  in  the  production  of  cheap  and  nutritious  food  neces- 
sarily directed  Rumford's  attention  to  fireplaces  and  chimney  flues. 
When  he  published  his  Essays  [1795-1800,  4  volumes],  in  London,  he 
reported  that  he  had  not  less  than  500  smoky  chimneys  on  his  hands."  — 
John  Tyndall,  New  Fragments,  New  York,  1892,  p.  123. 


HEAT  191 

an  Englishman.  In  1803  Eumford  went  to  France,  and  mar- 
ried the  widow  of  the  chemist  Lavoisier.  A  divorce  soon  fol 
lowed.     He  died  at  Auteuil,  near  Paris. 

Of  his  various  experiments,  the  ones  on  the  source  of  heat 
excited  by  friction,  published  in  1798,  are  of  the  greatest 
interest.     While  engaged  at  Munich  in  the  boring  of  cannon, 
he  was  surprised  at  the  heat  generated.     Whence  comes  this 
heat  ?     What  is  its  nature  ?     He  arranged  apparatus  so  that 
the  heat  generated  by  the  friction  of  a  blunt  steel  borer  raised 
the  temperature  of  a  quantity  of  water.     In  his  third  experi- 
ment,^ water  rose  in  one  hour  to  107°  F. ;  in  one  hour  and  a 
half  to  142°;   "at  the  end  of  two  hours  and  thirty  minutes 
the  water  actually  boiled!''     "It  is  difficult  to  describe  the 
surprise  and  astonishment,"  says  Eumford,  "  expressed  in  the 
countenances  of  the  bystanders,  on  seeing  so  large  a  quantity 
of  cold  water  (18 1  pounds)  heated,  and  actually  made  to  boil, 
without  any  fire  ...  yet  I  acknowledge  fairly  that  it  afforded 
me  a  degree  of  childish  pleasure,  which,  were  I  ambitious  of 
the  reputation  of  a  grave  philosopher,  I  ought  most  certainly 
rather  to  hide  than  to  discover."     The  source  of  heat  generated 
by  friction   "appeared   evidently  to   be   inexhaustible.''     The 
reasoning  by  which  he  concluded  that  heat  was  not  matter, 
but  was  due  to  motion,  we  can  give  only  in  part.     He  says, 
"  It  is  hardly  necessary  to  add,  that  anything  which  any  in- 
sulated body,  or  system  of  bodies,  can  continue  to  furnish  with- 
out limitation,  cannot  possibly  be  a  material  substance;  and  it 
appears  to  me  to  be  extremely  difficult,  if  not  quite  impossible, 
to  form  any  distinct  idea  of  anything  capable  of  being  excited 
and  communicated  in  the  manner  in  which  heat  was  excited 
and  communicated  in  these  experiments,  except  it  be  ^notion." 


1  The  Complete  Works  of  Count  Eumford,  published  by  the  American 
Academy  of  Arts  and  Sciences,  Boston,  Vol.  I.,  pp.  481-488. 


192  A  HISTORY   OF   PHYSICS 

In  1804  Eumford  wrote  in  a  letter  to  Marc  Auguste  Picteti 
of  Geneva,  "  I  am  persuaded  that  I  shall  live  a  sufficient  long 
time  to  have  the  satisfaction  of  seeing  caloric  interred  with 
phlogiston  in  the  same  tomb."  This  hope  was  hardly  realized. 
For  nearly  half  a  century  later  the  large  majority  of  phy- 
sicists and  chemists  continued  in  the  belief  that  heat  was  a 
substance. 

In  the  accurate  measurement  of  heat  Eumford  was  less  suc- 
cessful than  in  his  qualitative  work.  In  his  experiments  on 
heat  from  friction  he  himself  says  that  no  estimate  was  made 
of  the  heat  accumulated  in  the  wooden  box  holding  the  water, 
nor  of  that  dispersed  during  the  experiment.  From  Eum- 
ford's  data  we  may  make  a  rough  estimate  of  the  dynamical 
equivalent  of  heat.  He  estimated  the  thermal  capacity  of  the 
water  and  metal  as  the  equivalent  to  that  of  26.58  pounds  of 
water.  Enough  heat  could  be  generated  in  21  hours,  by  the 
use  of  one  horse,  to  raise  the  temperature  from  33°  to  212°  F. 
Hence,  the  rate  of  increase  in  temperature  was  1.2°  per  min- 
ute ;  the  number  of  calories  of  heat  generated  per  minute  was 
1.2  X  26.58,  or  31.92,  which  must  be  equivalent  to  one  horse- 
power, or  33,000  foot-pounds  per  minute.  Hence  one  calorie, 
using  Fahrenheit  degrees,  is  equivalent  to  1034  foot-pounds. 
Joule's  estimate  was  772  foot-pounds. 

Kumford's  conclusion  regarding  the  nature  of  heat  was 
vigorously  attacked  by  the  calorists,  but  it  was  confirmed  in 
1799  by  Sir  Humphry  Davy}  By  means  of  clockwork  he 
rubbed  two  pieces  of  ice  against  one  another  in  the  vacuum  of 
an  air-pump.  Part  of  the  ice  was  melted,  although  the  tem- 
perature of  the  receiver  was  kept  below  the  freezing-point. 
From  this  he  concluded  that  friction  causes  vibration  of  the 
corpuscles  of  bodies,  and  this  vibration  is  heat.     However,  he 

1  Davy's  Complete  WorkSj  Vol.  XL,  p.  11. 


HEAT  193 

was  not  so  confident  of  the  correctness  of  this  view  as  Rum- 
ford,  and  it  was  not  till  1812  that  he  felt  sure  in  asserting 
that  "the  immediate  cause  of  the  phenomenon  of  heat  is 
motion,  and  the  laws  of  its  communication  are  precisely  the 
same  as  the  laws  of  the  communication  of  motion."  ^  Argu- 
ing from  Eumford's  experiments,  a  conclusive  refutation  of 
the  caloric  theory  was  given  in  1807  by  Thomas  Young  in  his 
Natural  Philosophy.  But  Eumford,  Davy,  and  Young  made, 
at  the  time,  but  few  converts.^ 

An  important  observation  bearing  on  exact  thermometry  was 
announced  in  1822  by  Flaugergues,^  who  observed  the  gradual 
change  of  the  zero  point  of  mercury  in  glass  thermometers. 
Glass  does  not  immediately  return  to  its  original  volume  on 
cooling  from  a  high  temperature.  In  course  of  time  the 
capacity  of  the  bulb  diminishes  a  little,  rather  rapidly  at 
first,  and  then  very  slowly  for  years  afterwards.  This  prop- 
erty has  been  the  source  of  an  untold  amount  of  annoyance  to 
persons  aiming  to  secure  very  accurate  determinations  of  tem- 
perature. Joule  examined  the  "zero  reading"  of  a  delicate 
thermometer  at  intervals  over  a  period  of  thirty-eight  years, 
with  the  following  results :  April,  1844,  0°  F. ;  February,  1846, 
.42°;  January,  1848,  .51°;  April,  1848,  .53°;  February,  1853, 
.68°;  April,  1856,  .73°;  December,  1860,  .86°;  March,  1867, 
.90°;  February,  1870,  .93°;  February,  1873,  .94°;  January, 
1877,  .978°;  November,  1879,  .994°;  December,  1882,  1.020°.* 
Researches  on  glass,  carried  out  in  Europe,  have  resulted  in 
the  discovery  of  a  material  free  from  many  of  the  objections 
to  ordinary  glass,  and  the  accuracy  of  mercury  thermometers 

1  Davy,  Elements  of  Chemical  Philosophy^  p.  94. 

2  For  details  consult  G.  Berthold,  Bumford  und  d.  Mechanische 
Wdrmetheorie,  Heidelberg,  1875. 

^  Ann.  de  Chimie  et  de  Physique,  Vol.  21,  p.  333. 
*  Joule,  Scientific  Papers,  p.  558. 


194  A  HISTORY  OF  PHYSICS 

has  been  increased  fivefold.^  Wiebe  and  Scbott  of  Jena  have 
shown  that  glass  containing  either  sodium  or  potassium,  but 
not  both,  gives  the  least  displacement  of  the  zero.^ 

Recently  platinum  thermometers  have  been  recommended 
for  accurate  research.^  They  may  be  made  by  welding  a  coil 
of  line  platinum  wire  to  leads  of  relatively  low  electric  resist- 
ance. The  coil  and  leads  must  be  suitably  insulated  and 
supported.  It  is  an  improved  form  of  William  Siemens's 
electrical  pyrometer.  The  platinum  thermometer  shows  com- 
parative freedom  from  change  of  zero.  It  indicates  tempera- 
ture by  its  change  of  electrical  resistance,  which  is  always 
very  nearly  the  same  at  the  same  temperature.* 

The  first  careful  comparison  of  the  mercury  thermometer 
with  the  air  thermometer  was  made  in  1815  by  Dulong  and 
Petit.^  They  assumed  that  mercury  thermometers  agreed 
among  themselves,  so  that  a  table  of  corrections  carefully 
prepared  for  one  mercury  thermometer  was  applicable  to  all. 
That  this  is  not  the  case  was  shown  by  Regnault.^  Not  only 
have  different  kinds  of  glass  different  coefficients  of  expan- 
sion, but  they  have  different  laws  of  expansion.  More  than 
this,  I.  Pierre'^  has  shown  that  two  mercury  thermometers, 
made  of  the  same  piece  of  glass  with  equal  care,  did  not  agree 
exactly  with  each  other.     E,egnault  showed  that,  between  0° 

1  Nature,  Vol.  55,  1897,  p.  368.         2  jsfature,  Vol.  52,  1895,  p.  87. 

3  H.  L.  Callendak,  Phil.  Mag.  (5),  Vol.  32,  1891 ;  E.  H.  Griffiths, 
Science  Progress,  Vol.  II.,  1894-1895,  article,  "The  Measurement  of 
Temperature," 

*  For  detailed  information,  consult  "Metals  at  High  Temperatures," 
Nature,  Vol.  45,  1892,  pp.  534-540;  "Long-range  Temperature  and 
Pressure  Variables  in  Physics,*'  Science,  N.  S.,  Vol.  VI.,  1897,  pp.  338-356. 

^  Ann.  de  Chimie  et  de  Physique,  Vol.  2,  1815,  pp.  240-254  ;  reprinted 
in  OstwaWs  Klass.,  No.  44,  pp.  31-40. 

^  Ann.  de  Chimie  et  de  Physique  (3),  Vol.  5,  p  83  ;   Ostwald''s  Klass, 
No.  44,  pp.  164-181. 

7  Ann.  de  Chimie  et  de  Physique  (3),  Vol.  5,  1842,  p.  427. 


HEAT  195 

and  100°  C,  the  air  thermometer  and  the  mercury  thermometer 
of  ordinary  soft  glass  agree  very  closely,  though  at  about  the 
middle  of  the  scale  the  air  thermometer  lags  behind  the  other 
by  about  .2°.  Above  250°  C.  the  mercury  thermometer  reads 
considerably  higher;  at  300°  the  difference  is  1°;  at  350°  it 
is  3°.  Olzewski  showed  that  in  low  temperatures  hydrogen 
thermometers  are  still  quite  reliable ;  at  —  220°  C.  its  error  is 
not  more  than  1°.  In  recent  years,  perhaps  the  most  careful 
work  on  exact  thermometry  has  been  carried  on  in  connection 
with  determinations  of  the  mechanical  equivalent  of  heat,  by 
Eowland  and  others. 

Considerable  confusion  existed  at  one  time  in  the  minds  of 
leading  physicists,  and  still  exists  in  some  of  our  text-books, 
on  the  subject  of  temperature.  It  was  stated  as  a  merit  of 
the  mercurial  thermometer  that  mercury  "  expands  uniformly,^ 
or  of  the  air  thermometer  that  air  "expands  uniformly''^  or 
"  expands  nearly  uniformly,'^  and  yet  no  standard  of  reference 
was  given  by  which  this  uniformity  was  supposed  to  have 
been  established.  As  a  matter  of  fact,  we  may  take  any  sub- 
stance as  a  standard  and  then  define  equal  increments  of  tem- 
perature as  those  which  give  equal  increments  of  that  substance. 
But,  if  mercury  be  so  taken,  then  the  assertion  that  mercury 
expands  "uniformly"  is  destitute  of  good  sense.  Mercury 
gives  us  an  arbitrary  scale  of  temperature  differing,  probably, 
from  every  other  similar  scale.  Air  does  not  expand,  quite 
uniformly,  if  mercury  is  the  arbitrary  standard,  and  vice 
versa.  One  of  the  first  to  possess  clear  notions  on  this  sub- 
ject was  Lord  Kelvin,  who,  in  1848,  established  the  "  absolute 
thermodynamic  scale ''  of  temperature,^  which  is  independent 

1  William  Thomson,  Proc.  Cambridge  Phil.  Soc,  1848.  Consult  also 
his  article  "  Heat "  in  Encycl.  Brit.,  9th  ed.  For  a  "  Kritik  des  Tempera- 
turbegriffes,"  see  Mach,  Principien  d.  Wdrmelehre,  Leipzig,  1896,  pp. 
39-57. 


196  A   HISTORY   OF   PHYSICS 

of  the  particular  properties  of  any  particular  substance  and, 
therefore,  constitutes  a  much  more  satisfactory  foundation  for 
thermometry  than  any  arbitrary  scale.  It  is  now  our  ultimate 
scale  of  reference.  The  air  thermometer  gives  indications 
agreeing  very  closely  with  the  absolute  thermodynamic  scale. 

The  propagation  of  heat  in  solid  bodies  was  investigated 
mathematically  by  Joseph  Fourier  (1768-1830),  who  published, 
in  1822,  a  Avork  entitled,  La  Theorie  Analytique  de  la  Chaleur, 
which  not  only  marked  an  epoch  in  the  history  of  mathemati- 
cal physics,  but  stimulated  experimental  inquiry.  Eourier 
assumed  the  conductivity  of  a  substance  for  heat  to  be  con- 
stant for  all  temperatures.  But  James  David  Forbes  (1809- 
1868),  professor  at  Edinburgh,  showed  that  this  is  not  true, 
that  from  0°  to  100°  the  conductivity  of  iron  diminishes 
15.9%;  of  copper,  24.5%.  He  noticed  at  the  same  time  that 
there  was  an  accompanying  decrease  in  electric  conductivity. 

At  the  beginning  of  the  nineteenth  century,  the  laws  of 
gases  were  diligently  studied.  Amontons  had  arrived  at  an 
approximate  value  for  the  coefficient  of  expansion  of  air  under 
constant  pressure,  but  the  measurements  made  by  about  twenty 
physicists  of  the  eighteenth  century  differed  very  widely  from 
each  other.  The  first  to  deduce  the  law  as  we  now  know  it 
was  Jacques  Alexandre  Cesar  Charles  (1746-1823),  who  was 
professor  of  physics  at  the  Conservatoire  des  Arts  et  Metiers 
in  Paris.  He  was  best  known  for  his  improvements  in  the 
design  of  balloons.  At  his  suggestion,  hydrogen,  then  a  new 
gas  (discovered  by  Henry  Cavendish  in  1766),  was  used  in 
filling  balloons.  In  1773  the  two  brothers  Montgoljier  had 
raised  the  first  balloons  at  Annonay  in  southern  France,  and 
had  caused  a  great  sensation.  They  used  hot  air.  Charles, 
assisted  by  the  mechanic  Eobert,  in  1783,  raised  the  first 
hydrogen  balloon  from  the  Champ  de  Mars  in  Paris.  Later 
he  and  Eobert  made  balloon  ascensions  together. 


HEAT  197 

This  bold  navigator  of  the  air  busied  himself  also  with  the 
expansion  of  gases  and  discovered  what  is  known  as  the 
"  law  of  Charles  "  or  "  law  of  Gay-Lussac."  Charles  failed  to 
publish  his  results  and  it  was  only  by  accident  that  they 
became  known  to  Gay-Lussac.  Gay-Lussac's  researches  were 
published  in  1802.^  He  attributed  the  want  of  agreement  in 
earlier  experiments  to  the  presence  of  moisture.  His  own 
elaborate  investigation  led  him  to  the  conclusion  "that  in 
general  all  gases  by  equal  degrees  of  heat,  under  the  same 
conditions,  expand  proportionately  just  alike." 

Joseph  Louis  Gay-Lussac  (1778-1850)  was  educated  at  the 
Polytechnic  School,  became  assistant  to  the  chemist  Berthollet, 
and  later  professor  of  chemistry  at  the  Polytechnic  School,  of 
Physics  at  the  Sorbonne.  His  physical  researches  are  mainly 
on  the  expansion  of  gases.  To  ascertain  the  chemical  and 
electric  condition  of  the  air  in  the  upper  strata,  and  also 
to  measure  the  force  of  terrestrial  magnetism  at  great  eleva- 
tions, he  and  Biot  ascended  in  a  balloon  which  had  survived 
Napoleon's  campaign  in  Egypt.  "  Supplied  with  a  full  com- 
plement of  barometers,  thermometers,  hygrometers,  electrom- 
eters, and  instruments  for  measuring  magnetic  force  and 
dip,  as  well  as  frogs,  insects,  and  birds  for  galvanic  experi- 
ments, the  scientific  voyagers  embarked  on  August  23,  1804. 
They  began  their  experiments  at  an  altitude  of  6500  feet  and 
continued  them  to  the  altitude  of  13,000  feet,  and  with  a  suc- 
cess commensurate  with  their  wishes.  The  last  part  of  the 
excursion,  and  especially  the  landing  which  they  made,  was 
so  difficult  .  .  .  that  .  .  .  Biot,  though  a  man  of  activity  and 
not  deficient  in  personal  courage,  was  so  much  overpowered 
by  the  alarms  of  their  descent,  as  to  lose  for  a  time  the  entire 


1  Annales  de  Chimie,  VoL  43,  pp.  137-175 ;  Ostwald^s  Klass.,  No.  44, 
pp.  3-25. 


198  A  HISTORY   OF  PHYSICS 

possession  of  himself."  ^  Gay-Lussac  made  another  balloon 
excursion  the  same  year.  Air  bottled  at  a  height  of  6300 
metres  was  found  to  have  the  same  composition  as  air  near 
the  surface. 

The  expansion  of  gases  was  investigated  also  by  John 
Dalton  (1766-1844)  of  Manchester,  the  great  founder  of  the 
chemical  atomic  theory.^  His  conclusions  did  not  quite  agree 
with  Gay-Lussac's.  The  latter  had  shown  that,  using  a  mer- 
cury thermometer,  the  expansion  per  degree  is  a  constant 
fraction  of  the  volume  at  some  arbitrary  fixed  temperature. 
Dalton,  on  the  other  hand,  claimed  that  the  increment  of 
volume  for  each  equal  rise  of  temperature  is  a  constant  frac- 
tion of  the  volume  at  the  temperature  immediately  preceding. 
The  question  was  decided  by  Dulong  and  Petit  in  favour  of 
Gay-Lussac.^  The  value  of  the  coefficient  of  expansion  for 
the  interval  from  0""  to  100°,  as  determined  by  Gay-Lussac  and 
Dalton,  was  .375 ;  as  determined  in  1837  by  Fredrik  Eudberg 
(1800-1839),  professor  at  Upsala,  it  was  .365 ;  as  determined 
by  Magyius  and  BegnauU,  it  was  between  .366  and  .367.  The 
whole  subject  of  the  expansion  of  gases  was  independently 
re-investigated  with  the  aid  of  more  refined  methods  by  the 
two  experimentalists  last  named. 

We  pause  a  moment  to  catch  a  glimpse  of  some  of  the  men 
just  mentioned.  Alexis  TJierdse  Petit  (1791-1820)  was  professor 
of  physics  at  the  Polytechnic  School  in  Paris.  Pierre  Louis 
Dulong  (1785-1838)  held  the  same  position  for  some  years 
after  1820.  At  first  Dulong  practised  medicine,  but  as  he  not 
only  treated  the  poor  free  of  charge,  but  also  bought  medicine 
for  them,  he  found  his  vocation  too  expensive.    As  a  physicist, 

1  Proc.  of  the  Am.  Acad,  of  Arts  and  Sci.,  Vol.  VI.,  p.  20. 

2  Consult  H.  E.  EoscoE,  John  Dalton  and  the  Bise  of  Modern  Chem- 
istry, 1895. 

«  Ostwald's  Klass.y  No.  44,  pp.  28,  40. 


HEAT  199 

his  wealth  was  absorbed  by  the  cost  of  his  expensive  appara- 
tus.^  Most  of  his  researches  were  carried  on  in  conjunction 
with  other  men.  Some  were  carried  on  with  Petit ;  others  with 
Arago,  Berzelius,  Despretz.  Henri  Victor  Regnault  (1810- 
1878)  was  professor  in  Lyons,  then  in  Paris  at  the  Polytechnic 
School,  and  at  the  College  de  France.  After  1854  he  was 
director  of  a  porcelain  manufactory  at  Sevres.  He  displayed 
wonderful  patience  and  skill  in  the  execution  of  careful  meas- 
urements. His  numerical  tables  on  the  dilatation  of  elastic 
fluids,  on  the  elastic  force  of  steam,  on  the  heat  of  vaporization 
of  water,  on  the  specific  heat  of  water  at  different  temperatures, 
etc.,  still  rank  among  the  best.  But  he  lacked  that  creative 
genius  which  enables  its  possessor  not  only  to  experiment  but 
to  grapple  with  great  questions  of  theoretical  science.  Eeg- 
nault  proved  that  all  gases  do  not  possess  quite  the  same 
coefficient  of  expansion ;  that,  except  for  hydrogen,  it  increases 
with  the  initial  pressure ;  that  no  gas  obeys  Boyle's  law 
exactly.^ 

Eegnault's  experiments  showed  that,  as  the  pressure  in- 
creased, the  product  of  the  pressure  and  volume,  pv,  diminished 
in  all  gases  except  hydrogen.  If  Boyle's  law  were  followed, 
this  product  would  be  constant.  Eegnault's  observations  ex- 
tended over  a  comparatively  small  range  of  pressures.     That 

1  EOSENBERGER,  III.,  p.  221. 

2  Three  papers  by  Kegnault  and  two  by  Magnus,  on  the  expansion  of 
gases,  are  reprinted  in  Ostwald^s  Klass.,  No.  44.  Regnault's  most  valu- 
able experimental  results  are  collected  in  Vols.  21  and  26  of  the  Memoires 
of  the  French  Academy.  At  the  close  of  the  Franco-Prussian  War  Reg- 
nault found,  on  returning  to  his  laboratory  at  Sevres,  that  the  results  of 
his  last  great  research  on  the  phenomena  of  heat  accompanying  the  ex- 
pansion of  gases,  derived  from  over  600  observations,  had  been  destroyed. 
The  announcement  of  this  loss  was  his  last  communication  to  the  scien- 
tific world.  One  of  his  sons,  a  promising  artist,  died  on  the  battle-field. 
See  Nature,  Vol.  17,  1878,  pp.  263,  264. 


200  A    HISTORY   OF   PHYSICS 

pv,  as  p  increases,  does  not  diminish  for  all  pressures  beyond 
this  range,  but  reaches  a  minimum,  and  then  increases,  as  in 
case  of  hydrogen,  was  first  shown  by  the  Vienna  physician, 
Johann  August  batterer,  in  the  years  1850-1854,^  while  he  was 
endeavouring  to  liquefy  oxygen,  hydrogen,  and  air.  For  twenty 
years  this  interesting  observation  was  not  extended.  In  1870 
the  subject  was  taken  up  by  Cailletet  and  later  by  E.  H.  Amagat. 
The  experiments  of  the  latter  are  particularly  instructive.^ 
For  an  increase  of  pressure  from  30  to  320  atmospheres,  pv 
increases  continually  in  hydrogen ;  it  first  diminishes  a  trifle 
and  then  increases  in  nitrogen  (at  17.7°  C.) ;  it  diminishes 
greatly  and  then  increases  rapidly  in  ethylene  and  carbon 
dioxide.  The  variations  of  pv  are  very  rapid  near  the  critical 
point ;  they  are  more  pronounced  for  low  temperatures. 

The  first  important  work  on  the  liquefaction  of  gases  was 
performed  by  Faraday}  His  experiments,  begun  in  1823, 
showed  that  the  capability  of  being  liquefied  was  a  property 
common  to  most  gases.  A  bent  glass  tube  was  taken ;  into  its 
longer  leg,  which  was  closed,  there  was  introduced  a  substance 
which  would  evolve,  when  heated,  the  particular  gas  to  be 
tested.  The  shorter  leg  of  the  tube  was  then  sealed  up,  and 
cooled  by  being  placed  in  a  freezing  mixture.  When  the 
longer  leg  was  heated,  and  gas  was  generated,  the  pressure 
in  the  tube  increased,  and  in  many  cases  the  gas  condensed 
in  the  shorter  leg.     Thus,  by  heating  sodium  bicarbonate,  car- 

1  Fogg.  Ann.,  Vol.  62,  p.  139 ;  Vol.  94,  p.  436. 

2  Amagat,  Ann.  de  Chimie  et  de  Physique  (5),  Vol.  19,  1880,  p.  435, 
and  other  articles  in  the  same  journal.  Summaries  of  his  work  are 
found  in  Preston,  Theory  of  Heat,  London,  1894,  pp.  403-410  ;  Ostwald, 
Lehrb.  d.  Allgem.  Chemie,  Vol.  I.,  1891,  pp.  146-159. 

3  Before  Faraday,  several  gases  had  been  liquefied  by  cooling :  Marum, 
liquefied  ammonia ;  Mange  and  Clouet,  sulphurous  acid  ;  Northmore, 
chlorine  (in  1805);  Stromeyer,  arsenic  trihydride.  See  Ostwald,  op.  cit., 
Vol.  I.,  p.  294 ;  Nature,  Vol.  17,  1878,  p.  177. 


HEAT  201 

bonic  acid  gas  was  obtained,  wliicli  was  liquefied  in  tbe  short 
leg.  By  this  method  Faraday  liquefied  HjS,  HCl,  SO2,  CgNs? 
ISTHg,  CI2.  Thilorier  in  1835  produced  in  larger  quantities 
liquid  and  solid  CO^.  By  mixing  solid  CO2  with  ether,  he 
obtained  low  degrees  of  temperature  previously  undreamed 
of.  Notwithstanding  the  researches  of  Thilorier,  of  batterer, 
and  of  Faraday  in  1845,  several  gases  still  resisted  liquefaction. 
They  were  classed  under  the  name  of  '^permanent  gases,"  a 
title  which  they  bore  over  a  quarter  of  a  century,  until  1877. 

Meanwhile  new  things  were  being  ascertained  regarding  the 
continuity  of  the  gaseous  and  liquid  states  of  matter.  As 
early  as  1822  Charles  Cagniard-Latour  (1777-1859),  an  engi- 
neer, later  attache  to  the  Ministry  of  the  Interior  in  Paris, 
observed  that  ether,  alcohol,  and  water,  when  heated  in  her- 
metically sealed  tubes,  were  apparently  totally  changed  into 
vapour  occupying  only  from  two  to  four  times  the  original 
volume  of  the  liquid.  But  the  discovery  of  the  continuity  of 
the  liquid  and  gaseous  states  belongs  to  Tliomas  Andrews  (1813- 
1885),  the  vice-president  and  professor  of  chemistry  in  the 
college  at  Belfast.  In  his  experiments  pressure  was  produced 
by  screwing  up  mercury  into  a  capillary  tube,  in  which  the 
gas  was  kept  at  the  desired  temperature.  In  1863  he  wrote : 
"  On  partially  liquefying  carbonic  acid  by  piressure  alone,  and 
gradually  raising  at  the  same  time  the  temperature  to  88°  F. 
[30.92°  C],  the  surface  of  demarcation  between  the  liquid  and 
gas  became  fainter,  lost  its  curvature,  and  at  last  disappeared. 
The  space  was  then  occupied  by  a  homogeneous  fluid,  which 
exhibited,  when  the  pressure  was  suddenly  diminished  or  the 
temperature  slightly  lowered,  a  peculiar  appearance  of  moving 
or  flickering  striae  throughout  its  entire  mass.  At  tempera- 
tures above  88°  F.  no  apparent  liquefaction  of  carbonic  acid, 
or  separation  into  two  distinct  forms  of  matter,  could  be 
effected,  even  when  a  pressure  of  300  or  400  atmospheres  was 


202  A   HISTORY   OF   PHYSICS 

applied."  ^  This  temperature  of  30.92°  C,  at  wliicli  the  liquid 
and  the  gaseous  states  of  CO2  merge  into  one  another,  has  been 
called  by  Andrews  the  "critical  point."  Every  gas  has  its 
own  critical  temperature.  Below  this  the  substance  may  exist 
partly  as  a  vapour,  partly  as  a  liquid.  Above  it  this  is  not 
true ;  the  substance  may  be  made  to  pass  from  a  gas  to  a 
liquid  without  a  break  of  continuity,  so  that  it  is  impossible 
to  state  when  it  ceases  to  be  a  gas  and  begins  to  be  a  liquid. 
J.  D.  van  der  Waals  treated  the  subject  from  the  standpoint  of 
the  mathematical  theory  of  gases.  William  Ramsay,  in  1880, 
concluded  that  "  the  critical  point  is  that  point  at  which  the 
liquid  owing  to  expansion,  and  the  gas  owing  to  compression, 
acquire  the  same  specific  gravity,  and  consequently  mix  with 
each  other."  ^  Three  years  later  the  same  result  was  obtained 
by  Jules  Celestin  Jamin.^ 

Andrews  had,  in  1869,  Expressed  the  opinion  that  the  failure 
to  liquefy  the  "  permanent  gases  "  was  due  to  the  fact  that  their 
critical  temperatures  were  much  lower  than  the  lowest  tem- 
perature hitherto  obtained.  Taking  this  hint,  two  young 
investigators  —  Pictet  and  Cailletet  —  made  the  year  1877 
memorable  in  the  history  of  science  by  their  brilliant  demon- 
stration that  the  "permanent  gases"  may  be  liquefied,  and 
that  molecular  cohesion  is  a  property  of  all  bodies  without 
exception.  The  means  at  the  command  of  both  experimenters 
arose  from  their  industrial  equipments,  one  for  making  iron, 
the  other  for  making  ice.  L.  Cailletet,  one  of  the  greatest  iron- 
masters of  France,  employed  the  enormous  resources  at  his 
disposal  at  the  Chatillon-sur-Seine  in  Paris.*  Raoul  Pictet  of 
Geneva  was  interested  in  the  artificial  production  of  ice,  and 

1  Miller's  Chemical  Physics^  3d  ed.,  p.  328. 

2  liature,  Vol.  22,  1880,  p.  46. 

3  Compt.  Bend.,  Vol.  96,  1883,  p.  1448. 
^Naturcy  Vol.  17,  1878,  pp.  177,  178. 


HEAT  203 

now  has  a  laboratory  in  Berlin  for  experimentation  on  low 
temperatures.  Low  temperatures  find  industrial  application ; 
for  instance,  in  the  purification  of  chloroform.  On  December 
24,  1877,  at  the  same  meeting  of  the  French  Academy,  it  was 
announced  that,  working  independently  and  by  different 
methods,  Cailletet  and  Pictet  had  liquefied  oxygen.  A  week 
later  Cailletet  performed  a  series  of  experiments  in  the  labora- 
tory  of  the  Ecole  Normale  at  Paris  in  the  presence  of  leading 
French  scientists.  He  then  and  there  liquefied  hydrogen,  nitro- 
gen, and  air.  The  same  result  was  achieved  by  Pictet. 
Cailletet's  process  consists  in  compressing  the  gas  into  a  small 
tube,  cooling  it,  and  then  suddenly  allowing  it  to  expand  by 
removal  of  the  pressure.  This  instantaneous  expansion  of  the 
gas  causes  such  a  low  degree  of  cold  that  a  large  portion  of 
the  gas  is  condensed  into  a  cloud  of  vapour.  In  case  of  oxygen 
the  temperature  of  the  tube  was  reduced  to  —  29°  C.  by  the 
application  of  sulphurous  acid.  The  pressure  was  300  atmos- 
pheres. The  sudden  expansion  probably  lowered  the  tem- 
perature as  much  as  200°.-^  Pictet  used  more  elaborate 
apparatus  (costing  about  50,000  francs),  and  obtained  the 
condensed  gases  on  a  larger  scale.  The  low  temperatures 
were  obtained  on  the  principle  of  evaporation.  A  vacuum 
pump  withdrew  from  a  tube  the  vapour  above  liquid  sulphurous 
acid;  the  vapour  was  then  liquefied,  cooled,  and  returned  to 
the  tube.  Thus  a  complete  circulation  was  maintained.  In 
this  way  the  temperature  of  the  liquid  fell  to  about  —  70°  C. 
Within  this  tube  was  another  thinner  tube  containing  liquid 
carbonic  acid.  The  object  of  the  former  liquid  was  to  keep 
the  latter  cool.  The  carbonic  acid  was  let  into  another  tube, 
where  its  temperature  was  reduced  by  evaporation,  brought 


1 A  large  figure  of  Cailletet's  apparatus  will  be  found  in  Nature^ 
Vol.  17,  p.  267. 


204  A  HISTORY  OF  PHYSICS 

about  as  before  by  a  vacuum  pump,  to  —  140°  C.  The  vapour 
of  CO2  was  condensed  by  the  sulphurous  acid.  Thus  by  com- 
pression, liquefaction,  and  exhaustion,  there  was  a  circulation 
of  CO2  like  that  of  sulphurous  acid.  A  tube  containing  oxygen 
passed  inside  the  tube  of  solidified  CO2  at  — 140°.  The 
oxygen  was  generated  by  heating  chlorate  of  potash  in  a  strong 
shell  at  one  end  of  the  last-mentioned  tube.  The  other  end  of 
that  tube  was  furnished  with  a  stop-cock.  The  oxygen  was  con- 
densed in  the  tube  by  the  combined  action  of  its  own  pressure 
of  several  hundred  atmospheres  and  the  intense  cold.  On  open- 
ing the  stop-cock  a  small  stream  of  oxygen  escaped.  In  its 
central  portion  it  was  white,  indicating  the  liquid  or  solid  con- 
dition. In  case  of  hydrogen,  the  escaping  stream  was  steel-blue. 
In  larger  quantities  the  three  gases,  oxygen,  nitrogen,  and 
hydrogen,  have  been  liquefied  by  Sigmund  v.  Wrohlewski  (1848- 
1888)  and  Karl  Olszewski  of  the  University  of  Cracow  in 
Austria-Hungary,  and  by  James  Dewar  of  the  Eoyal  Institu- 
tion in  London.  Their  apparatus  is  based  on  the  general 
principle  of  that  designed  by  Pictet,  only  it  has  been  found 
better  to  use  other  liquids,  like  ethylene  or  oxygen.  Olzewski 
determined  critical  points,  boiling-points,  freezing-points, 
and  densities.  He  found  boiling-points  as  follows :  oxygen, 
-182.7°  C. ;  argon,  -187°;  nitrogen,  -194.4°;  hydrogen, 
—243.5°.  Freezing-points:  argon,  —189.6°;  nitrogen,  —214°.^ 
James  Dewar  succeeded  in  1898  in  obtaining  liquid  hydrogen 
in  larger  quantities  (half  a  wine  glass),  and  in  liquefying 
helium.  In  1891  he  announced  that  liquid  oxygen  and  liquid 
ozone  are  attracted  by  the  poles  of  a  magnet.  J.  Dewar  and 
J.  A.  Fleming  have  examined  the  electric  resistance  of  metals 
at  low  temperatures.  The  resistance  of  some  pure  metals 
(platinum,  for  instance)  diminishes  at  low  temperatures  at 

1  See  table  in  Nature,  Vol.  61,  1895,  pp.  355,  356. 


HEAT  205 

such  a  rate  that,  if  the  rate  is  kept  up  in  the  lower  tempera- 
tures not  yet  reached,  it  will  vanish  at  the  absolute  zero. 

The  study  of  phenomena  of  heat  led  to  a  better  compre- 
hension of  meteorological  phenomena.  At  one  time  it  was 
believed  that  dew  fell  from  the  stars,  or,  at  any  rate,  from 
great  heights.  The  first  scientific  study  of  the  formation  of 
dew  was  made  by  the  London  physician,  William  Charles  Wells 
(1757-1817),  and  the  results  published  in  his  Essay  on  Dew, 
1814.  In  a  clear,  quiet  night,  the  grass  radiates  heat  into 
free  space,  whence  no  heat  returns.  Being  a  poor  conductor, 
the  lower  parts  of  the  grass  receive  little  heat  from  the  earth. 
The  grass  cools  and  vapour  condenses  upon  it.  Good  con- 
ductors, like  metals,  receive  heat  from  surrounding  bodies, 
and,  therefore,  are  not  covered  with  dew.  A  cloudy  sky 
hinders  the  formation  of  dew  by  returning  the  radiated  heat. 
Winds  are  unfavourable,  because  they  carry  heat  to  the  cool- 
ing objects.  Wells  supposed  that  only  a  very  small  part  of 
the  dew  deposited  comes  from  vapour  rising  from  the  earth 
or  the  evaporation  from  plants.  Recent  investigations  by 
Badgeley^  and  R.  BusselP  have  shown  that  Wells  underesti- 
mated the  role  played  by  both  the  earth  and  the  plant.  The 
vapour  has  been  shown  by  experiment  to  come  largely  from  the 
earth  beneath,  and  not  from  the  air  above.  This  is  contrary 
to  the  commonly  received  view. 

New  information  has  been  obtained  in  recent  years  on  the 
formation  of  rain  and  fog,  which  has  hardly  yet  found  its  way 
into  elementary  text-books.  It  has  been  shown  by  Coulier, 
E.  Mascart,^  and  especially  by  John  Aitken,^  that  in  the  de- 

1  Froc.  of  the  Boyal  Meteor.  Soc,  April,  1891. 

2  Nature,  Vol.  47,  1892,  pp.  210-213. 

3  Naturforscher,  1875,  p.  400  ;  Journal  de  Pharmacie  et  de  Chimie 
(4),  Vol.  22,  p.  165;  Nature,  Vol.  23,  1881,  p.  337. 

*  Nature,  Vol.  23,  p.  196  ;  Vol.  41,  p.  408  ;  Vol.  44,  1891,  p.  279 ;  Vol 
45,  p.  299  ;  Vol.  49,  1893,  p.  544. 


206  A   HISTOKY   OF  PHYSICS 

velopment  of  fogs  and  clouds,  the  presence  of  dust  is  essential. 
That  is,  "  whenever  water  vapour  condenses  in  the  atmosphere, 
it  always  does  so  on  some  solid  nucleus ; "  "  dust  particles  in 
air  are  nuclei  on  which  vapour  condenses  ;  "  except  for  dust  we 
should  have  no  fogs,  no  clouds,  no  mists,  and  probably  no  rain. 
City  fogs  are  due  to  dust.  It  is  not  true  that  dust  is  always 
absolutely  essential  for  cloudy  condensation;  it  may  be 
brought  about  by  the  presence  of  hydrochloric,  sulphuric,  or 
nitric  acids,  or  by  very  high  degrees  of  supersaturation.  Yet 
the  conditions  in  the  atmosphere  are  such  that,  except  for 
dust,  we  could  hardly  have  rain.  Aitken  invented  a  dust 
counter ;  he  made  extensive  observations  in  England  and  in 
Switzerland.  By  observations  on  the  Bigi  he  concluded  "  that 
whenever  a  cloud  is  formed,  it  at  once  begins  to  rain,  and  the 
small  drops  fall  into  the  drier  air  underneath,  where  they  are 
evaporated,  the  distance  to  which  they  will  fall  depending  on 
their  size  and  the  dryness  of  the  air.'' 

One  of  the  earliest  recorded  experiments  on  the  heating  and 
cooling  of  a  gas,  by  condensation  and  rarefaction,  respectively, 
was  made  by  a  workman  in  a  French  gun  factory,  who  ignited 
tinder  by  compression  of  air.  An  account  of  this  experiment 
was  sent  to  Paris  by  Mollet,  professor  at  Lyons.  The  subject 
was  carefully  examined  by  John  Dalton,  who,  in  1800,  read 
a  paper  "on  the  heat  and  cold  produced  by  the  mechanical 
condensation  and  rarefaction  of  air.''  ^ 

The  science  of  thermodynamics  had  its  origin  in  attempts 
to  determine  mathematically  how  much  work  can  be  gotten 
out  of  a  steam-engine.  The  first  impulse  to  this  was  given  by 
Nicolas  Leonard  Sadi  Carnot  (1 796-1832),  who,  in  1824,  pub- 
lished his  Rejlexions  sur  la  puissance  motrice  du  feu}    Carnot 

1  KOSENBERGER,  III.,  p.  224. 

2  Reprinted  in  German  in  OstwaWs  Klass.,  No.  37  ;  an  Englisli  trans- 
lation by  R.  H.  Thurston  appeared  in  1890. 


HEAT  207 

introduced  the  consideration  of  cyclic  operations,  in  wliicli  a 
working  substance,  after  a  series  of  changes,  is  brought  back  to 
its  initial  condition.  He  also  advanced  the  principle  of  revers- 
ibility, by  which  the  heat  may  be  taken  from  the  condenser 
and  restored  to  the  source  by  the  expenditure  of  an  equal 
quantity  of  work.  Assuming  perpetual  motion  to  be  impossi- 
ble, he  concluded  that  no  engine  can  have  a  greater  efficiency 
than  a  reversible  engine.  At  this  time  Carnot  was  an  adherent 
of  the  caloric  theory ;  he  believed  in  the  doctrine  of  the  con 
servation  of  caloric;  he  compared  the  motive  power  of  heat 
with  that  of  falling  water.  Both,  he  says,  have  a  maximum 
power,  independent,  in  one  case,  of  the  machine  upon  which 
the  water  acts,  and,  in  the  other  case,  of  the  nature  of  the 
substance  receiving  the  heat.  The  motive  power  of  water  de- 
pends upon  the  amount  of  water  and  the  height  through  which 
it  falls ;  the  motive  power  of  heat  depends  upon  the  quantity 
of  caloric  and  the  difference  in  temperature  between  the  source 
and  the  receiver.  But  some  years  later  Carnot  became  con- 
vinced of  the  falsity  of  the  caloric  theory.  His  later  writings, 
which  have  remained  unpublished  until  recent  times,  prove 
that  he  was  finally  persuaded  of  the  truth  of  the  dynamical 
theory  of  heat.  More  than  this,  he  had  grasped  the  law  of 
the  conservation  of  energy.  "  Motive  power  is  in  quantity  in- 
variable in  nature;  it  is,  correctly  speaking,  never  either  pro- 
duced or  destroyed." 

Though  the  importance  of  Carnot's  work  of  1824  was  empha- 
sized by  B.  P.  E.  Clapeyron,  it  did  not  meet  with  general 
recognition  until  it  was  brought  forward  by  William  Thomson^ 
who  pointed  out  the  necessity  of  modifying  Carnot's  reasoning 
so  as  to  bring  it  into  accord  with  the  new  theory  of  heat.  In 
1848  Thomson  showed  that  Carnot's  principle  of  cyclic  trans- 
formations leads  to  the  conception  of  an  absolute  thermodynamic 
scale  of  temperature.     In  1849  he  published  "an  account  of 


208  A   HISTORY   OF   PHYSICS 

Carnot's  theory  of  the  motive  power  of  heat,  with  numerical 
results  deduced  from  Regnault's  experiments."  In  February, 
1850,  Rudolph  Clausius  (1822-1888)  communicated  to  the  Ber- 
lin Academy  a  paper  on  the  same  subject,  which  contains  the 
Protean  second  law  of  thermodynamics  :  "  Heat  cannot,  of 
itself,  pass  from  a  colder  to  a  hotter  body."  Clausius  was 
at  this  time  professor  in  Zurich ;  later  he  went  to  Wtirzburg, 
and,  after  1869,  was  at  Bonn.  He  was  no  great  experimenter, 
but  ranked  very  high  as  a  mathematical  physicist.^  In  the 
same  month  of  February,  1850,  William  John  M.  Hankine 
(1820-1872),  professor  of  engineering  and  mechanics  at  Glas- 
gow, read  before  the  Eoyal  Society  of  Edinburgh  a  paper  in 
which  he  declares  heat  to  consist  in  the  rotational  motion  of 
molecules,  and  arrives  independently  at  some  of  the  results 
reached  previously  by  Clausius.  He  does  not  mention  the 
second  law  of  thermodynamics,  but  in  a  subsequent  paper  he 
declares  that  it  can  be  derived  from  equations  contained  in 
his  paper.  His  proof  of  the  second  law  is  not  free  from 
objections.  In  March,  1851,  there  appeared  a  paper  by  William 
Thomson  which  contained  a  perfectly  rigorous  proof  of  the 
.  second  law.  He  obtained  it  before  he  had  seen  the  researches 
of  Clausius.  The  statement  of  this  law,  as  given  by  Clausius, 
has  been  much  criticised,  particularly  by  Eankine,  Theodor 
Wand,  P.  G.  Tait,  and  Toiver  Preston.  Repeated  efforts  to 
deduce  it  from  general  mechanical  principles  have  remained 
fruitless.  The  science  of  thermodynamics  was  developed  with 
great  success  by  Thomson,  Clausius,  and  Eankine.  As  early 
as  1852  Thomson  discovered  the  law  of  the  dissipation  of 
energy,  deduced  at  a  later  period  also  by  Clausius. 

1  During  the  Franco-Prussian  War,  his  burning  patriotism  did  not  per- 
mit him  to  stay  at  home.  He  undertook  the  leadership  of  an  ambulance 
corps,  which  he  formed  of  Bonn  students.  See  Proc.  of  Boy.  Sac.  of 
London,  Vol.  48,  1890,  p.  II. 


HEAT  209 

The  first  law  of  thermodynamics  is  merely  the  application 
of  the  principle  of  the  conservation  of  energy  to  heat-effects. 
This  principle  is  the  greatest  generalization  in  physics  of  the 
nineteenth  century.  Its  history  is  remarkable  from  various 
points  of  view.  Several  thinkers  arrived  at  this  great  truth 
at  about  the  same  time ;  and,  at  first,  all  of  them  were  either 
met  with  a  very  cold  reception  or  were  completely  ignored. 
The  principle  of  the  conservation  of  energy  was  established 
by  the  Heilbronn  physician,  Robert  Mayer,  and  again  indepen- 
dently by  Ludwig  August  Colding  of  Copenhagen,  Joule  in 
England,  and  Helmholtz  in  Germany. 

Robert  Mayer  (1814-1878)  was  born  in  Heilbronn.  In  the 
gymnasium  and  the  theological  school  he  gave  no  evidence 
of  great  intellectual  power.  In  1832  he  entered  upon  the 
study  of  medicine  at  Tubingen,  and  in  1838  began  to  practise, 
but  he  never  found  the  work  of  a  practising  physician  agree- 
able to  his  tastes.  He  travelled  considerably  and  engaged  in 
the  study  of  physiology.  An  observation  made  in  1840,  on  the 
blood  of  a  patient  in  a  tropical  climate,  was  the  origin  of  his 
scientific  writings.  It  led  him  to  the  study  of  those  physical 
forces  on  which  the  phenomena  of  vitality  depend.  Thus, 
he  was  led  from  the  contemplation  of  organic  nature  to  the 
preparation  of  a  paper,  "  On  the  Forces  of  Inorganic  Nature," 
1842.  It  was  refused  publication  in  Poggenclorff^s  Annalen, 
but  was  accepted  by  Liebig  for  the  May  number  of  his 
Annalen.  It  attracted  no  attention,  though  containing  the 
great  principle  that  the  energy  of  the  world  is  constant.  A 
second  paper,  1845,  could  be  published  only  at  his  own 
expense.  Several  other  papers  were  published  later.^  The 
following  story,  related  by  Mach,  shows  Mayer's  alertness  of 

1  See  J.  J.  Wetrauch,  Bobert  Mayer^  Stuttgart,  1890.  Consult  also 
Weyrauch,  Die  MechaniJc  derWdrmevon  Robert  Mayer,  3d  ed.,  1893; 
\Yeyradch,  Kleinere  Schriften  und  Briefe  von  Bobert  3Iayer,  1893. 


210  A   HISTORY    OF   PHYSICS 

mind :  ^  "  During  a  hurried  meeting  with  Mayer  in  Heidelberg 
once,  Jolly  remarked,  with  a  rather  dubious  implication,  that  if 
Mayer's  theory  were  correct,  water  could  be  warmed  by  shak- 
ing. Mayer  went  away  without  a  word  of  reply.  Several  weeks 
later  ...  he  rushed  into  the  latter's  presence  exclaiming:  'Es 
ischt  so  ! '  (It  is  so,  it  is  so !)  It  was  only  after  considerable 
explanation  that  Jolly  found  out  what  Mayer  wanted  to  say." 
The  mind  of  E.obert  Mayer  became  seriously  affected  by  the 
lack  of  appreciation  of  his  ideas,  by  controversies  regarding  his 
rights  of  priority,  as  well  as  by  the  death  of  two  of  his  chil- 
dren. On  May  28,  1849,  he  unsuccessfully  attempted  suicide 
by  jumping  from  a  second-story  window.  After  a  seeming 
recovery,  he  wrote  a  paper  on  the  mechanical  equivalent  of 
heat.  In  1851  he  was  placed  in  an  insane  asylum,  where  he 
was  cruelly  treated.  In  1853  he  was  set  free,  but  he  never 
again  regained  complete  mental  equilibrium.  In  1858  a  few 
voices  were  heard  in  Germany  in  praise  of  Mayer,  but  the  one 
who  did  most  to  bring  him  historical  justice  was  John  Tyndall, 
who  in  1862  lectured  before  the  Eoyal  Institution  on  Robert 
Mayer  and  also  translated  several  of  Mayer's  papers.  William 
Thomson  and  Tait,  placing  a  much  lower  estimate'  on  Mayer's 
researches,  brought  the  charge  that  Tyndall  was  belittling  the 
work  of  Joule.^ 

James  Prescott  Joule  (1818-1889)  was  born  at  Salford,  near 
Manchester,  where  he  was  the  proprietor  of  a  large  brewery. 
At  an  early  age  he  engaged  in  electromagnetic  researches. 
After  laborious  tests  he  succeeded  in  showing  that  during 
electrolytic  action  there  was  an  absorption  of  heat  equivalent 
to  the  heat  evolved   during  the  original  combination  of  the 

1  "On  the  Part  played  by  Accident  in  Invention  and  Discovery," 
Monist,  Vol.  6,  1896,  p.  171. 

2  Consult  "Notes  on  Scientific  History"  by  Tyndall  in  Phil.  Mag.^ 
July,  1864 


HEAT  211 

constituents  of  the  compound  body.  He  studied  tlie  relations 
between  electrical,  chemical,  and  mechanical  effects,  and  was 
led  to  the  great  discovery  of  the  mechanical  equivalent  of 
heat.  In  a  paper  read  before  the  British  Association,  in  1843, 
he  gave  the  number  as  460  kilogramme-metres.  Friends  who 
recognized  the  physicist  in  the  young  brewer  persuaded  him 
to  become  a  candidate  for  the  professorship  of  natural  phi- 
losophy at  St.  Andrews,  Scotland,  but  his  slight  personal 
deformity  was  an  objection  in  the  eyes  of  one  of  the  electors, 
and  he  did  not  receive  the  appointment.  He  remained  a 
brewer,  but  continued  scientific  research  throughout  life. 
In  April,  1847,  Joule  gave  a  popular  lecture  in  Manchester, 
delivering  "  the  first  full  and  clear  exposition  of  the  universal 
conservation  of  that  principle  now  called  energy."^  The 
local  press  would  at  first  have  nothing  to  do  with  it.  One 
paper  refused  to  give  even  a  notice  of  it;  the  Mmichester 
Courier,  after  long  debate,  published  the  address  in  full.  In 
June,  1847,  the  subject  was  presented  before  the  British 
Association  meeting  at  Oxford.  The  chairman  suggested 
that  the  author  be  brief;  no  discussion  was  invited.  In  a 
moment  the  section  would  have  passed  on  to  other  matters 
without  giving  the  new  ideas  any  consideration,  "  if  a  young 
man  had  not  risen  in  the  section,  and  by  his  intelligent 
observations  created  a  lively  interest  in  the  new  theory.  The 
young  man  was  William  Thomson."  The  result  was  that  the 
paper  caused  a  great  sensation ;  Joule  had  attracted  the  atten- 
tion of  scientific  men.  After  the  meeting  Joule  and  Thomson 
discussed  the  subject  further,  and  the  latter  "obtained  ideas 
he  had  never  had  before,"  while  through  him  Joule  heard  for 
the  first  time  of  Carnot's  theory.^ 

1  A.  W.  RtJcKER  in  Fortnightly  Beview,  1894,  p.  652.  We  are  making 
considerable  use  of  this  article,  entitled,  "Hermann  von  Helmholtz." 
It  is  reprinted  in  Smithsonian  Beport,  1894. 

2  Nature,  Vol.  49,  1893,  p.  164. 


212  A   HISTORY   OF   PHYSICS 

Joule  experimented  on  the  mechanical  equivalent  of  heat 
for  about  forty  years.  By  magneto-electric  currents  he  got, 
in  1843,  the  value  of  460  kilogramme-metres  as  the  equivalent 
of  the  large  French  calorie.  By  the  friction  of  water  in 
tubes,  he  got  424.9 ;  by  the  compression  of  air,  in  1845,  443.8  •, 
by  the  friction  of  water  he  got,  in  1845,  488.3 ;  in  1847,  428.9 ; 
in  1850,  423.9 ;  in  1878,  423.9.^ 

The  mechanical  equivalent  of  heat  is  such  an  important 
constant  in  nature  that  several  physicists  since  Joule  have 
thought  it  desirable  to  redetermine  it.  One  of  the  most 
accurate  determinations  was  made  in  1879  by  Henry  A.  Row- 
land of  Baltimore.^  The  part  of  the  work  which  received 
greater  attention  than  Joule  had  given  it,  was  the  subject  of 
themometry.  Joule  used  mercury  thermometers.  Eowland, 
for  convenience,  used  a  mercury  thermometer  too,  but  com- 
pared it  with  an  air  thermometer  and  then  reduced  his  data 
to  the  absolute  scale.  Eowland  paid  attention  also  to  varia- 
tions in  the  specific  heat  of  water  for  different  temperatures. 
Starting  with  the  water  at  different  temperatures,  he  obtained 
by  friction  of  water  in  a  calorimeter  different  values  for  the 
mechanical  equivalent.  This  variation  in  the  values  he  at- 
tributed to  changes  in  the  specific  heat  of  water.  The  latter 
was  found  by  him  to  reach  a  minimum  at  30°  C.  More 
recently  the  mechanical  equivalent  of  heat  has  been  measured 
by  D'Arsonval,  Miculescu,  E.  H.  Griffiths,  and  others.  Joule's 
estimate  of  this  constant  has  been  raised  somewhat  by  the 
later  determinations.^ 

The  same  year,  1847,  in  which  Joule  announced  his  views 

1  Consult  The  Scientific  Papers  of  James  Prescott  Joule,  in  two  volumes, 
London,  1884  ;  Nature,  Vol.  43,  1890,  p.  112. 

2  Proc.  of  the  Am.  Acad,  of  Arts  and  Sciences,  N.  S.,  Vol.  7,  1880. 

3  Consult  E,  H.  Griffiths  in  Science  Progress,  Vol.  1,  1894,  p.  127 ; 
Johns  Hovkins  Circulars,  1898,  No.  135. 


HEAT  213 

on  energy,  Helmholtz  read  before  the  Physical  Society  in 
Berlin  a  paper  on  the  same  subject.  Hermann  von  Helmholtz 
(1821-1894)  was  born  at  Potsdam,  studied  medicine  in  Berlin, 
became  assistant  at  the  charity  hospital  there,  then  military 
surgeon  in  Potsdam  (1843-1847),  teacher  of  anatomy  at  Ber- 
lin, of  physiology  at  Konigsberg,  later  at  Bonn  and  Heidelberg 
(1858-1871).  In  1871  he  accepted  the  chair  of  physics  at  the 
University  of  Berlin.  He  possessed  an  intellect  of  extraor- 
dinary breadth  and  depth.  He  was  of  the  first  rank  as  a 
physiologist,  as  a  physicist,  and  as  a  mathematician.  A  few 
years  ago  W..,  K.  Clifford,  in  his  article  "  Seeing  and  Think- 
ing," spoke  of  him  as  follows :  ^^  In  the  first  place  he  began  by 
studying  physiology,  dissecting  the  eye  and  the  ear,  and  find- 
ing out  how  they  acted,  and  what  was  their  precise  constitu- 
tion ;  but  he  found  that  it  was  impossible  to  study  the  proper 
action  of  the  eye  and  ear  without  studying  also  the  nature  of 
light  and  sound,  which  led  him  to  the  study  of  physics.  He 
had  already  become  one  of  the  most  accomplished  physiolo- 
gists of  this  century  when  he  commenced  the  study  of  physics, 
and  he  is  now  one  of  the  greatest  physicists  of  this  century. 
He  then  found  it  was  impossible  to  study  physics  without 
knowing  mathematics ;  and  accordingly  he  took  to  studying 
mathematics  and  he  is  now  one  of  the  most  accomplished 
mathematicians  of  this  century." 

His  famous  paper  on  energy,  entitled  "  Die  Erhaltung  der 
Kraft,"  ^  which  the  youth  of  twenty-six  read  before  the  Physical 
Society  of  Berlin  in  1847,  was  at  first  looked  upon  as  a  fan- 
tastic speculation.  The  editor  of  Poggendorff's  Annalen,  who 
in  1843  declined  Mayer's  paper,  rejected  Helmholtz's  also. 
As    Joule    had    been    supported    by    William    Thomson,    so 


1  Eeprinted  in  OstwaWs  Klass.,  No.  1.     In  Note  5  Helmholtz  outlines 
the  history  of  the  new  principle  of  energy. 


214  A    HISTORY    OF   PHYSICS 

Helmlioltz  was  defended  by  his  fellow-student  Du  Bois- 
Beymond  and  by  the  mathematician  C.  G.  J.  Jacobi.  Helm- 
holtz's  paper  was  published  in  pamphlet  form  in  1847.  For 
a  time  it  attracted  little  notice,  but  in  1853  it  was  vigorously 
attacked  by  Clausius.  Later  it  subjected  its  author  to  viru- 
lent attacks  from  Eugen  Karl  Dlihring  and  others,  who  ac- 
cused him  of  being  a  dishonest  borrower  from  his  forerunner, 
Robert  Mayer.^  In  1847  Helmholtz,  like  Joule,  had  not 
heard  of  Robert  Mayer,  but  later  he  cheerfully  acknowledged 
Mayer's  priority. 

By  the  word  Kraft,  used  by  Mayer  and  Helmholtz,  we  must 
understand  energy.  For  a  time  great  confusion  existed  in 
text-books  between  the  terms  "force"  and  '^energy."  The 
two  terms  were  frequently  used  synonymously  and  continue 
to  be  so  used  by  some  psychological  writers.  The  use  of  the 
word  "energy"  to  denote  the  quantity  of  work  which  a 
material  system  can  do  was  introduced  by  Thomas  Young  in 
Lecture  VIII.  of  his  Natural  Philosophy.  With  him  it  desig- 
nated mv^;  Lord  Kelvin  in  1849  used  it  for  ^mv^.  The 
expression  "conservation  of  energy"  is  due  to  Rankine.^ 

1  Physical  Bevieic,  Vol.  2,  1894,  p.  224. 

2  Students  desiring  more  detailed  information  on  heat  may  consult, 
besides  Rosenbekger,  Poggendorfe,  and  Heller,  Mach,  Principien 
der  Wdrmelehre,  Leipzig,  1896  ;  Georg  Helm,  Lehre  von  der  Energie 
historisch-kritisch  Entwickelt,  Leipzig,  1887  ;  M.  P.  Desains,  Bapport 
sur  les  progres  de  la  theorie  de  la  chaleur,  Paris,  1868  ;  M.  Bertin, 
Bapport  sur  les  progres  de  la  thermodynamique  en  France,  Paris,  1867  ; 
Joseph  Peveling,  Gesch.  d.  Gesetze  von  d.  Erhaltung  d.  Materie  und 
Energie^  Aachen,  1891. 


ELECTJRICITY   AND   MAGNETISM  215 

ELECTEICITY   AND  MAGNETISM 

Electrical  progress,  both  theoretical  and  practical,  has  been 
so  rapid  that  this  has  been  called  the  age  of  electricity. 

After  the  discovery  of  current  electricity  by  Galvani  and 
the  construction  of  the  voltaic  pile,  Carlisle  and  Nicholson 
decomposed  water  by  low  pressure  currents.  This  feat  caused 
great  excitement.  In  September,  1800,  Joliann  Willielm  Hitter 
(1776-1810),  of  Silesia,  announced  that  he  had  succeeded  in 
collecting  the  two  gases  separately  and  that  copper  oould  be 
precipitated  from  blue  vitriol. 

Sir  Humphry  Davy  (1778-1829)  was  among  the  early 
workers  in  this  line.  While  a  poor  boy,  Davy  attained 
notoriety  for  being  "  so  fond  of  chemical  experiments."  After 
serving  as  assistant  in  the  Pneumatic  Institution  at  Bristol, 
he  in  1801  became  lecturer  in  chemistry  at  the  Eoyal  Insti- 
tution in  London.  His  lectures  delighted  the  fashionable 
audiences.  Said  Coleridge,  "I  go  to  Davy's  lectures  to  in- 
crease my  stock  of  metaphors."  It  has  been  said  that,  if 
Davy  had  not  been  one  of  the  first  chemists,  he  would  have 
been  one  of  the  first  poets  of  his  age.^ 

Davy  showed  that  in  the  decomposition  of  water,  the  volume 
of  hydrogen  is  double  that  of  oxygen.  His  most  striking 
discoveries  were  the  resolution,  by  electrolysis,  of  the  fixed 
alkalies,  potash  and  soda.  In  1807  the  elements  sodium 
and  potassium  were  thus  discovered,  and  the  rapid  advance 
of  chemistry  was  aided  by  electricity. 

The  apparent  migration  of  the  products  of  electric  decompo- 
sition called  forth  several  curious  theories,  but  the  one  which 
held  its  ground  for  over  half  a  century  and  is  still  described 
in  text-books  was  proposed  by  Ch.  J.  D.  von  Grothuss^  (1785- 

1  The  Gallery  of  Portraits,  with  Memoirs,  London,  1883,  Vol.  L,  p.  12. 

2  Or  Grotthuss. 


216 


A   HISTOKY    OF   PHYSICS 


1822).  In  his  boyhood  Grothuss  was  forbidden  the  study  ol 
chemistry,  but  later  he  pursued  scientific  courses  at  Leipzig, 
Paris  (at  the  Polytechnic  School),  and  Naples.  After  1808 
he  lived  on  his  estate  in  Lithauen,  Prussia,  giving  his  leisure 
time  to  chemical  research.  During  his  last  years  he  suffered 
intensely  from  some  organic  trouble  which  finally  drove  him 

to  suicide.  He  is  best 
known  by  his  paper, 
first  published  at 
Rome  in  1805,  when 
he  was  only  twenty 
years  old,  "Memoire 
sur  la  decomposition 
de  I'eau  et  des  corps, 
qu'elle  tient  en  dis- 
solution, a  I'aide  de 

„     ..  Pelectricite      ^alvan- 

FiG.  14.  ^ 

ique."  ^  In  a  quantity 
of  water  (Fig.  14),  composed  of  oxygen  (marked  — )  and  hydro- 
gen (marked  +),  electric  polarity  manifests  itself  as  soon  as  an 
electric  current  is  established  in  the  water.  All  oxygen  atoms 
in  the  path  of  the  current  receive  a  tendency  to  move  toward 
the  positive  pole,  while  all  hydrogen  atoms  in  the  same  path 
tend  toward  the  negative  pole.  Consequently,  if  the  molecule 
oil  gives  off  its  oxygen  o  to  the  positive  wire,  then  the  hydrogen 
li  soon  gets  oxidized  by  the  arrival  of  another  oxygen  atom  o', 
whose  hydrogen  7i'  combines  with  ?-,  etc.  The  same  action,  in 
the  opposite  sense,  occurs  in  the  molecule  QP.  Thus,  there  is 
a  progressive  alternate  separation  and  union  of  atoms.  That 
such  separation  and  recombination  should  go  on,  without  the 

1  Reprinted  in  1806  in  Annates  de  Chimie.  Vol.  58,  pp.  54-74,  A  Ger- 
man translation  in  full  is  given  in  Ostwald,  JElektrochemie,  Leipzig,  1896, 
pp.  309-316. 


ELECTRICITY   AND   MAGNETISM  217 

expenditure  of  work,  is  contrary  to  the  laws  of  energy.  As 
Ostwald  puts  itj  energy  in  a  condition  of  rest  cannot  of  itself 
become  active;  a  stone  lying  on  the  ground  cannot  of  itself 
rise  and  then  fall  again.  Grothuss  supposed  "that  at  the 
moment  of  the  segregated  appearance  of  the  hydrogen  and 
oxygen,  there  takes  place  a  division  of  their  natural  electricity, 
either  by  their  contact  or  by  mutual  friction,  so  that  the  former 
assumes  the  positive,  the  latter  the  negative  condition."  On 
this  point  the  first  differences  of  opinion  arose.  Modified 
hypotheses  were  suggested  by  H.  G.  Magnus  of  Berlin,  by  the 
Swede  chemist  Jons  Jacob  Berzelius  (1779-1848),  and  by 
Auguste  Arthur  de  la  Rive  (1801-1873)  of  Geneva.  As  to  the 
mechanism  of  electrolytic  conduction,  the  molecular  chain  of 
Grothuss  was  modified  somewhat  by  Faraday  and  W.  Hittorf 
(born  1824),  professor  of  physics  at  Mtinster.  But  the  first 
radical  modification  of  Grothuss's  theory  was  made  by  Clausius 
in  1857.^  He  argued  that,  according  to  the  electrolytic  theories 
then  held,  the  E.  M.  F.  first  turns  the  molecules,  so  that  the 
positive  ions  face  the  kathode  and  the  negative  ions  the  anode, 
and  then  pulls  asunder  the  ions  which  were  previously  firmly 
united  in  the  molecule.  Now,  to  separate  these  ions  requires 
a  force  of  definite  intensity.  Hence,  if  the  electrolytic  force 
acting  upon  the  ions  is  less  than  the  attraction  between  the 
ions,  there  can  be  no  segregation  whatever;  if  this  force 
increases,  many  molecules  will  be  broken  up  at  once.  This  is 
contrary  to  fact.  Experiment  shows  that  the  weakest  E.  M.  E. 
causes  decomposition,  and  that  the  action  is  proportional  to 
the,  intensity  of  the  current.  To  remove  this  difficulty, 
Clausius  assumed  that  the  ions  are  not  permanently  united 
with  each  other ;  that  part  of  them  exist  in  the  liquid  in  an 
uncombined  state    wandering  about   seeking  partners.      The 

1  Fogg.  Ann.,  Vol.  101,  p.  338. 


218  A    HISTORY    OF   PHYSICS 

electromotive  force  of  the  current  acts  upon  these  loose  atoms. 
Some  of  the  ions  being  free  to  begin  with,  the  weakest  current 
can  act.  Thus  Clausius  advanced  the  idea  of  dissociation  to 
explain  electrolysis.  This  dissociation  hypothesis  was  modified 
by  G.  Quincke  of  Heidelberg  to  better  explain  the  migration 
of  ions.  It  was  used  by  F.  Kohlrausch  (born  1840 ;  formerly 
professor  at  Strassburg,  now  Helmholtz's  successor  at  the 
Eeichsanstalt  in  Berlin)  to  explain  the  facts  of  electrolytic 
conductivity.  Yet  Clausius's  dissociation  theory  of  the  con- 
stitution of  electrolytes  met  with  little  favour  until  1887,  when 
Svante  Arrhenius  in  Stockholm  brought  new  arguments  to  sup- 
port it,  based  on  J.  H.  vanH  Hoff^s  theory  of  solutions  and  the 
phenomena  of  osmotic  pressure.^  From  certain  new  considera- 
tions the  conclusion  was  reached  that  in  solutions  there  exists 
a  partial  dissociation  of  the  dissolved  substances.^  Important 
researches  on  this  subject  have  been  carried  on  by  Wilhehn 
Ostwald  of  Leipzig,  and  Walter  Nernst  of  Gottingen. 

In  these  researches  there  are  exemplified  the  beneficial 
results  arising  from  an  intimate  reunion  of  the  two  branches 
of  science,  physics  and  chemistry.  At  the  beginning  of  the 
century  many  scientists  contributed  original  researches  in  both 
sciences ;  they  were  chemists  as  well  as  physicists.  But  about 
1835  a  separation  took  place ;  men  were  known  only  as  physi- 
cists or  only  as  chemists.  About  1885,  after  half  a  century 
of  separation,  a  tendency  to  reunion  became  apparent  in  what 
is  known  as  the  "  Leipzig  School,"  with  Ostwald,  Nernst,  and 
Arrhenius  at  the  head. 

One  of  the  most  interesting  results  achieved  by  this  school 
is  the  solution  of  a  problem  which  has  been  under  discussion  a 

1  Arrhenius,  Zeitschr.  f.  Physik.  Chemie,  Vol.  1,  p.  631. 

2  For  a  systematic  and  historical  exposition  of  osmotic  pressure  and 
the  theory  of  solutions,  see  W.  Ostwald,  AUgemeine  Chemie,  Vol.  1, 
1891,  Viertes  Buch,  or  P.  Muir's  English  translation  of  the  same. 


ELECTRICITY   AND   MAGNETISM  219 

whole  century,  viz.  the  source  or  seat  of  the  electromotive 
force  in  a  voltaic  cell.  It  will  be  remembered  that  Volta's 
contact  theory  did  not  meet  with  general  acceptation.  The 
theory  that  the  origin  and  maintenance  of  the  power  of  the 
voltaic  pile  resided  in  the  contact  of  different  metals  was 
opposed  by  Giov.  Vol.  Mattia  Fabbroni  (1752-1822)  of  Florence, 
Italy;  by  WoUaston  in  England,  and  by  Bitter  in  Germany. 
They  held  that  the  real  source  of  voltaic  electricity  was 
chemical  action.  This  view  was  taken  also  by  A.  G.  Becquerel 
in  Paris,  A.  A.  de  la  Rive  of  Geneva,  and  particularly  by 
Faraday  in  London,  who,  in  1837  and  1840,  published  many 
experiments  which  seemed  to  disprove  the  contact  theory. 
Volta^s  contact  theory  received  strongest  support  in  Germany. 
It  was  advocated  by  Fechner,  Poggendorff,  C.  H.  Pfaff,  Ohm, 
and  others.  When  the  principle  of  the  conservation  of  energy 
was  established,  this  theory,  as  originally  taught,  had  to  be 
modified  ;  the  mere  contact  of  metals  could  not  give  rise  to  an 
inexhaustible  supply  of  electric  energy.  It  became  evident 
that  in  the  voltaic  cell  there  took  place  a  transformation  of 
energy.  Nevertheless,  the  seat  of  the  electromotive  force 
might  still  be  at  the  points  of  metallic  contact.  The  question 
appears  at  last  to  have  been  settled  in  a  paper  by  Walter 
Nernst  on  the  electromotive  action  of  ions.^  The  seat  of  the 
electromotive  force  coincides  with  the  seat  of  the  chemical 
phenomena,  and  lies  in  the  surfaces  of  contact  between  the 
metals  and  the  electrolytes.  Nernst  has  established  the  fol- 
lowing fundamental  formula: 

^  =  ^nat.  log-, 
V 

where  E  is  the  potential  difference  between  the  metal  and 
electrolyte  under  consideration;  p  is  the  osmotic  pressure  of 

1  Zeitschr.  f.  Physik.  Chemie,  Vol.  4,  1889,  p.  129. 


220  A   HISTORY   OF   PHYSICS 

the  metal  ions  in  the  solution ;  ^  is  a  constant  depending  on 
the  units  used ;  P  is  a  constant  of  integration,  to  be  physically 
interpreted  as  a  pressure.  This  theory  of  the  voltaic  cell  rests 
on  vanH  Hoff^s  ideas  of  osmotic  pressure  and  the  views  of 
Arrhenius  on  dissociation  in  electrolytes.-*  Nernst  gives  the 
following  comparison  with  the  action  of  a  Daniell  cell :  '^  Given 
a  reservoir  containing  liquid  carbonic  acid  and  another  con- 
taining a  substance,  for  instance,  caustic  potash,  absorbing  this 
one  rapidly,  and  between  the  two  a  cylinder  and  piston  con- 
trivance to  turn  the  difference  of  pressure  into  work.  The 
machine  does  work  until  all  the  carbonic  acid  is  absorbed; 
just  so  a  Daniell  cell  acts  till  the  zinc  is  used  up." 

Volta's  pile  and  crown  of  cups,  or  slight  modifications 
thereof,  were  for  a  long  while  the  only  means  known  for  the 
generation  of  current  electricity.  They  laboured  under  the 
defect  of  a  rapid  diminution  in  current  on  account  of  polariza- 
tion. The  amalgamation  of  the  zincs,  first  practised  by 
Sturgeon  in  1830,  was  a  step  in  advance.  A  year  previous 
A.  C.  Becquerel  constructed  a  cell  yielding  a  somewhat  steadier 
current.  A  glass  trough  was  divided  into  three  partitions  by 
two  layers  of  gold-beater's  skin.  The  middle  portion  between 
the  two  membranes  was  filled  with  a  salt ;  into  the  outer  por- 
tions containing  appropriate  solutions,  dipped  copper  and  zinc 
plates,  respectively.  With  such  a  cell  a  tangent  galvanometer 
deflection  of  84°  fell  to  6S°  in  half  an  hour.  Better  success  in 
inventing  a  constant  cell  crowned  the  efforts  of  John  Frederic 
Daniell  (1790-1845),  who  was  professor  of  chemistry  at  King's 
College,  London.  He  contributed  to  science  not  only  the 
'^  Daniell  cell,"  but  also  "  Daniell's  hygrometer."     The  inven- 

1  For  an  exposition  of  the  theory  see  W.  Ostwald,  Elektrochemie^ 
1896,  pp.  1133-1148;  W.  Ostwald,  Allg.  Chemie,  Vol.  2,  I.,  1893;  W. 
Nernst,  Theoretical  Chemistry,  trans,  by  C.  S.  Palmer,  1895,  pp.  609- 
616 ;  A.  WiJLLNER,  Experimentalphysik,  Vol.  3,  1897,  pp.  909-919. 


ELECTRICITY   AND   MAGNETISM  221 

tion  of  the  cell,  in  1836,  grew  out  of  his  contact  with  Faraday. 
In  a  letter  describing  the  cell  he  wrote  Faraday  as  follows : 
"  You  know  how  deep  an  interest  I  have  taken  in  your  Experi- 
mental Researches  in  Electricity,  and  how  zealously  I  have 
availed  myself  of  the  opportunities  which  you  have  ever  kindly 
offered  me,  of  profiting  by  your  oral  explanation  of  such  diffi- 
culties as  occurred  to  me  in  the  study  of  your  last  series  of 
papers."^  In  his  original  cell  the  concentrated  copper  sulphate 
and  dilute  sulphuric  acid  were  separated  from  each  other  by 
an  animal  membrane  —  the  windpipe  of  an  ox.  Soon  after 
J.  P.  Gassiot  suggested  the  use  of  an  earthen  porous  cup  in 
place  of  the  windpipe.  In  1839  Sir  William  Robert  Grove 
(1811-1896)  communicated  to  the  British  Association  a  paper, 
entitled  "  On  a  Small  Voltaic  Battery  of  Extraordinary  Energy,'' 
and  exhibited  a  battery  "  hastily  constructed."  In  1840  Grove 
was  appointed  professor  of  experimental  philosophy  at  the 
London  Institution.  Later  he  entered  upon  the  practice  of 
law,^  but  retained  his  interest  in  science.  Still  another  battery,^ 
in  which  the  polarization  was  prevented  mechanically  by  giving 
the  electronegative  plate  a  rough  surface,  was  designed  by  the 
London  surgeon,  Alfred  Smee*  (1818-1877).  In  Grove's  cell 
the  great  cost  of  the  platinum  was  an  objection ;  so  Bunsen  and 

1  Phil.  Trans.,  Part  I.,  1836,  p.  107. 

"^Electrician  (London),  Vol.  37,  1896,  p.  483;  Nature,  Vol.  54,  1896, 
p.  393. 

3  Phil.  Mag.  (3),  Vol.  16,  1840. 

*  The  reader  may  be  interested  in  the  following  jew  d"* esprit,  being  part 
of  an  electric  valentine,  written  by  Clerk  Maxwell : 

"  Constant  as  Daniell,  strong  as  Grove  ; 

Ebullient  through  all  its  depths  like  Smee ; 
My  heart  pours  forth  its  tide  of  love. 
And  all  its  circuits  close  in  thee.". 

All  four  stanzas  are  given  in  L.  Campbell  and  W.  Garnett,  Life  oj 
J.  a  Maxwell,  1882,  p.  630. 


222  A   HISTORY   OF   PHYSICS 

others  suggested  the  use  of  carbon  in  place  of  the  platinum. 
Descriptions  of  "  Bunsen's  cell "  appeared  in  1841.^  Among  the 
numerous  open  circuit  batteries  of  the  present  time  a  prominent 
one  is  that  brought  forth  in  1867  by  Georges  Ledanche  (1839- 
1882),  a  Parisian  chemist.  A  cell  whose  electromotive  force  is 
even  more  constant  than  that  of  the  Daniell  was  suggested  in 
1873  by  Latimer  Clark,  modified  forms  of  which  have  been 
used  by  Lord  Eayleigh,  Helmholtz,  and  Henry  S.  Carhart.  The 
Clark  cell  has  been  adopted  as  the  international  standard  of 
electromotive  force,  and  official  specifications  for  its  preparation 
have  been  issued. 

In  1803  Bitter  described  the  first  secondary  or  storage 
battery.  He  found  that  when  two  platinum  wires  were 
dipped  in  water  and  a  battery  current  passed  through  so 
that  hydrogen  appeared  at  one  wire  and  oxygen  at  the  other, 
then,  if  the  wires  Avere  disconnected  from  the  battery  and  con- 
nected with  each  other  by  a  conductor,  the  two  wires  acted 
like  the  plates  of  a  battery,  and  a  current  passed  for  a  short 
time  in  this  new  circuit.  Its  direction  was  opposite  to  that  of 
the  original  current.  The  subject  was  studied  in  1843  by 
Grove,  who  constructed  a  gas  battery  to  illustrate  the  phe- 
nomenon of  "polarization."  In  1859  Gaston  Plante  (1834- 
1889),  a  pupil  of  A.  C.  Becquerel,  made  a  thorough  study  of 
this  method  of  storing  energy,  and  devised  a  secondary  cell 
consisting  of  two  pieces  of  sheet  lead  rolled  up  and  dipping 
into  dilute  sulphuric  acid.  The  lead  plates  had  to  be  "  formed  " 
(coated  at  the  anode  with  a  semi-porous  film  of  dioxide  of  lead, 
and  at  the  kathode  with  a  spongy  metallic  surface)  by  sending  a 
current  through  the  cell,  and  reversing  its  direction  several 
times.  His  cell  had  a  higher  electromotive  force  than  any 
primary  battery,  nevertheless  it  hardly  reached  commercial 

'^Pogg.  Ann.,  Vol.  54,  1841,  p.  417. 


ELECTKICITY    AND   MAGNETISM  223 

efficiency  on  account  of  the  tedious  process  of  "forming." 
Little  attention  was  paid  to  it.  The  operation  of  "  forming  " 
was  avoided  in  1881  by  Camille  A.  Faure}  This  was  accom- 
plished by  coating  the  lead  plates  with  red  lead.  Thereby 
the  capacity  of  the  cell  was  also  increased.  After  this  im- 
provement commercial  circles  suddenly  became  interested. 
Four  cells  were  sent  from  Paris  to  London  in  1881  weighing 
only  75  pounds,  yet  it  was  said  they  were  charged  with 
IjOOOjOOO  foot-pounds  of  energy !  After  all,  this  was  no  more 
than  the  energy  stored  up  in  a  few  ounces  of  coal.  Filled 
with  hope,  inventors  put  forth  extraordinary  effort  to  make 
storage  batteries,  or  "accumulators,"  commercially  available. 
While  they  are  now  used  quite  extensively,  nevertheless  the 
results  are  disappointing. 

The  science  of  electromagnetism  originated  in  1819  in  what 
is  known  as  "  Oersted's  experiment."  Hans  Christian  Oersted 
(1777-1851)  was  born  at  Eudkjobing,  Langeland,  attended 
the  University  of  Copenhagen,  and  later  was  professor  at  the 
university  and  polytechnic  school  there.  Eegarding  Oersted's 
great  discovery,  Hansteen  wrote  Faraday  in  1857  as  follows  :  ^ 
"  Already  in  the  former  century  there  was  a  general  thought 
that  there  was  a  great  conformity,  and  perhaps  identity, 
between  the  electrical  and  magnetical  force;  it  was  only  the 
question  how  to  demonstrate  it  by  experiments.  Oersted 
tried  to  place  the  wire  of  his  galvanic  battery  perpendicular 
(at  right  angles)  over  the  magnetic  needle,  but  marked  no 
sensible  motion.  Once,  after  the  end  of  his  lecture,  as  he 
had  used  a  strong  galvanic  battery  in  other  experiments,  he 
said :  ^  Let  us  now  once,  as  the  battery  is  in  activity,  try  to 
place  the  wire  parallel  with  the  needle ; '  as  this  was  made,  he 
was  quite  struck  with  perplexity  by  seeing  the  needle  making 

1  Bom  1840,  died  1898. 

2  B.  Jones,  Life  and  Letters  of  Faraday,  London,  1870,  Yol.  II.,  p.  390. 


224  A   HISTORY   OF   PHYSICS 

a  great  oscillation  (almost  at  right  angles  with  the  magnetic 
meridian).  Then  he  said:  ^Let  us  now  invert  the  direction  of 
the  current/  and  the  needle  deviated  in  the  contrary  direction. 
Thus  the  great  detection  was  made ;  and  it  has  been  said,  not 
without  reason,  that  ^he  tumbled  over  it  by  accident.'  He 
had  not  before  any  more  idea  than  any  other  person  that  the 
force  should  be  transversal.  But  as  Lagrange  has  said  of 
I^ewton  on  a  similar  occasion :  '  Such  accidents  only  meet 
persons  who  deserve  them.' "  ^ 

"  Professor  Oersted  was  a  man  of  genius,  but  he  was  a  very 
unhappy  experimenter ;  he  could  not  manipulate  instruments. 
He  must  always  have  an  assistant,  or  one  of  his  auditors  who 
had  easy  hands,  to  arrange  the  experiment ;  I  have  often  in 
this  way  assisted  him  as  his  auditor."  ^ 

Oersted  placed  different  media  between  the  needle  and  the 
wire  carrying  the  current,  and  concluded  that  the  current  ^^acts 
upon  the  needle  through  glass,  metals,  wood,  water,  resin, 
earthen  jars,  stones ;  for  when  we  placed  between  the  two  a 
plate  of  glass,  or  of  metal,  or  a  board,  the  result  was  not 
cancelled ;  indeed  all  three  combined  hardly  lessened  the 
effect." 

A  In  1876  H.  A.  Rowland  showed*  that  a  magnetic  needle  is  affected 
also  by  a  rotating  body  carrying  an  electrostatic  charge.  The  latter  acts 
like  a  true  current. 

2  For  Oersted's  paper  see  GilherVs  Ann.,  Vol.  66,  1820,  p.  295;  Ost- 
WALD,  Elektrochemie,  1896,  p.  367;  OstwalcVs  Klass.,  No.  63.  E.  A.  P. 
Barnard  said  of  this  discovery:  "  "When  Oersted,  in  1819,  observed  the 
disturbance  of  the  magnetic  needle  by  the  influence  of  a  neighbouring 
magnetic  current,  how  wild  and  visionary  would  not  that  have  been  pro- 
nounced to  be,  who  should  have  professed  to  read,  in  an  indication  so 
slight,  the  grand  truth  that  science  had,  that  day,  stretched  out  the  scep- 
tre of  her  authority  over  a  winged  messenger,  whose  fleetness  should 
make  a  laggard  even  of  Oberon's  familiar,  sprite,  and  render  the  velocity 
which  could  'put  a  girdle  round  the  earth  in  forty  minutes'  tardy  and 
unsatisfactory  ?  " 


ELECTRICITY   AND   MAGNETISM  225 

Oersted's  experiment  was  repeated  everywhere.  Dominique 
Frangois  Jean  Arago  (1786-1853),  the  noted  Parisian  astrono- 
mer and  physicist,  observed  the  following  year  (1820)  that  iron 
filings  were  attracted  by  the  current.  He  concluded  that  the 
wire  carrying  the  current  must  be  considered  a  magnet,  even 
if  it  is  not  of  iron.  In  1822  Davy  proved  that  this  apparent 
attraction  of  the  filings  was  really  due  to  their  peripheral 
arrangement  around  the  wire ;  opposite  poles  of  the  filings 
attracting  each  other  and  establishing  a  chain  around  the  wire. 
The  fact  that  the  magnetizing  force  acts  in  a  plane  at  right 
angles  to  the  wire  induced  Ampere  to  twist  the  wire  into  a 
spiral  in  order  to  intensify  the  effect  upon  a  needle  placed 
inside.  Andre  Marie  Amph'e  ^  (1775-1836)  was  born  at  Lyons, 
and  early  displayed  mathematical  power.  During  the  Revolu- 
tion his  father  was  beheaded.  In  consequence,  young  Ampere 
was  mentally  crushed ;  hour  after  hour  was  passed  in  silence, 
while  he  was  staring  into  the  sky,  or  mechanically  heaping 
sand  into  little  piles.  After  a  year  he  awoke  from  his  mental 
stupor  and  his  love  for  science  was  rekindled  by  his  reading 
Eousseau's  work  on  botany.  After  his  marriage  in  1799,  his 
religious  emotions  became  very  strong.  His  intense  Cathol- 
icism, though  weakened  in  the  middle  of  his  career,  again 
asserted  itself  later  in  life.  He  became  professor  of  physics 
and  chemistry  at  Lyons.  After  his  wife's  death.  Ampere, 
depressed  and  melancholy,  wished  to  leave  Lyons.  "  Ampere, 
celebrated,  overwhelmed  with  honourable  distinctions,  the  great 
Ampere !  apart  from  his  mental  labours,  became  once  more 
hesitating  and  fearful,  uneasy  and  troubled,  and  more  disposed 
to  place  confidence  in  others  than  in  himself."  ^    In  1805  he 

^Consult  Arago,  "Eulogy  on  Ampfere,"  Smithsonian  Heport,  1872, 
p.  111. 

2  The  Story  of  his  Love,  being  the  journal  and  early  correspondence  oj 
Andre  Marie  Ampere,  edited  by  Madame  H.  C,  London,  1873,  p.  164. 


226  A   HISTORY   OF    PHYSICS 

became  connected  with  the  Polytechnic  School  in  Paris,  where, 
for  twenty  years,  he  engaged  in  important  researches.^ 

While  Oersted  had  discovered  simply  the  action  of  a  current 
on  a  magnet.  Ampere  discovered  the  action  of  a  current  upon 
another  current :  parallel  currents  in  the  same  direction  attract 
each  other  j  those  in  opposite  directions  repel  each  other.^  In 
these  beautiful  phenomena  some  critics  saw  nothing  more 
than  the  old  electric  attractions  and  repulsions.  To  this 
Ampere  replied  that  while  equal  electric  charges  repel  each 
other,  conductors  carrying  parallel  currents  attract  each  other. 
Another  critic  aimed  to  belittle  the  discovery  by  asserting 
that,  since  it  was  known  that  two  currents  acted  upon  one  and 
the  same  magnet,  it  was  evident,  to  begin  with,  that  they 
would  act  upon  each  other.  Upon  hearing  this,  Arago  drew 
two  keys  out  of  his  pocket  and  replied,  "Each  of  these  keys 
attracts  a  magnet;  do  you  believe  that  they,  therefore,  also 
attract  each  other  ?  " 

Ampere  gave  a  rule  —  "Ampere's  rule"  —  for  the  direction 
in  which  a  magnet  is  deflected  by  a  current.     Paraday  arrived 


1  The  following  extract  from  a  letter  of  1805  gives  a  vivid  picture  of 
the  man  :  "  My  life  is  a  circle,  with  nothing  to  break  its  uniformity.  .  .  . 
I  have  but  one  pleasure,  a  very  hollow,  very  artificial  one,  and  which  I 
rarely  enjoy,  and  that  is  to  discuss  metaphysical  questions  with  those  who 
are  engaged  in  this  science  at  Paris,  and  who  show  me  more  kindness 
than  the  mathematicians.  But  my  position  obliges  me  to  work  at  the 
pleasure  of  the  latter,  a  circumstance  which  does  not  contribute  to  my 
diversion,  for  I  have  no  longer  any  relish  for  mathematics.  Nevertheless, 
since  I  have  been  here  I  have  written  two  treatises  on  Calculation  which 
are  to  be  printed  in  the  journal  of  the  Polytechnic  School.  It  is  seldom, 
except  on  Sunday,  that  I  can  see  the  metaphysicians,  such  as  M.  Maine 
de  Biran,  with  whom  I  am  very  intimate,  and  M.  de  Tracy,  with  whom 
I  dine  occasionally  at  Auteuil,  where  he  resides.  It  is  almost  the  only 
place  in  Paris  where  the  country  reminds  me  of  the  banks  of  the  Sadne." 
—  Ibidem,  p.  322. 

2  Annales  de  Chimie  et  de  Physique,  Vol.  15,  1820. 


ELECTRICITY   AND   MAGNETISM  227 

at  a  more  compreliensive  conception  of  the  relation,  and 
devised  experiments  showing  that  current  and  magnet  have 
a  tendency  to  encircle  each  other.  This  result  was  extended 
by  Ampere.  Contrary  to  the  opinion  of  TJiomas  Johanyi  See- 
heck,  who  looked  upon  the  electric  current  as  a  magnetic 
action,  Ampere  considered  a  magnet  as  primarily  due  to 
the  action  of  electric  currents.  Each  particle  in  a  magnet 
has  an  equatorial  current,  producing  magnetic  poles.  To 
magnetize  a  magnet  is  to  cause  all  these  hypothetical  mo- 
lecular currents  to  flow  in  the  same  direction.  Terrestrial 
magnetism,  according  to  Ampere,  is  due  to  electric  currents 
around  the  earth.  In  1823  Ampere  published  a  paper  giving 
a  mathematical  theory  of  the  new  phenomena.  Maxwell 
describes  this  research  as  "perfect  in  form  and  unassailable 
in  accuracy." 

Georg  Simon  Olim'^  (1789-1854)  was  an  ingenious  investi- 
gator who,  although  removed  from  the  influence  of  personal 
contact  with  the  great  physicists  of  his  time,  yet  working 
independently  and  alone,  discovered  the  great  law  bearing 
his  name.  He  was  born  in  Erlangen,  attended  the  university 
at  his  native  place,  then  taught  school  at  Gottstadt,  ISTeufchatel, 
and  Bamberg.  At  the  age  of  thirty  he  became  teacher  oi 
mathematics  and  physics  at  the  gymnasium  in  Cologne.  He 
taught  there  nine  years  with  great  success.  A  pupil  of  that 
time,  who  attained  great  celebrity  as  a  mathematician,  was 
Lejeune  Dirichlet.  Ohm  became  ambitious  to  engage  in 
research,  but  the  want  of  leisure  and  books,  as  well  as  the 
lack  of  suitable  apparatus,  rendered  progress  difficult.  The 
mechanical  skill  which  he  had  acquired  as  a  boy  through  his 
father,  a  locksmith,  enabled  him  to  construct  much  apparatus 


1  Consult  Eugene  Lommel,   "The  Scientific  Work  of  George  Simon 
Ohm"  in  Smithsonian  Beport,  1891,  pp.  247-256. 


228  A   HISTORY   OF   PHYSICS 

for  himself.  His  first  experiments  ^  were  on  the  relative  con- 
ductivity of  metals.  Taking  wire  of  different  material,  but 
of  the  same  thickness,  he  found  that  the  following  lengths 
possessed  equal  conductivities :  copper  1000,  gold  574,  silver 
356,  zinc  333,  brass  280,  iron  174,  platinum  171,  tin  168,  lead 
97.  Observe  that  his  measurements  made  silver  a  much 
poorer  conductor  than  copper,  though  it  is  actually  a  better 
conductor.  Later,  Ohm  tried  to  verify  his  results  and  found 
the  mistake.  The  silver  wire  first  used,  in  being  drawn, 
became  covered  with  oily  leather,  so  that,  while  both  wires 
were  drawn  so  as  to  be  apparently  of  equal  thickness,  the 
first  one  was  really  much  thinner.  Further  experiments  with 
wires  of  the  same  material,  but  of  different  thicknesses,  yielded 
him  the  result  that  they  have  the  same  conductivity  if  their 
lengths  are  proportional  to  their  cross-sections.  In  these  tests 
he  was  greatly  troubled  by  variations  in  his  batteries  ("  Wogen 
der  Kraft").  Finally,  at  the  suggestion  of  Poggendorff,  he 
adopted  thermo-electric  elements  as  the  sources  of  current. 
These  were  free  from  this  source  of  trouble. 

In  the  experiments  by  which  Ohm  established  his  law,  he 
used  two  tin  vessels  A  and  B,  Fig.  15.  In  A  he  kept  boiling 
water ;  in  B  snow  or  ice.  He  prepared  a  bar  of  bismuth  abb'a' ; 
to  this  he  fastened  by  screws  strips  of  copper,  whose  two 
free  ends  dipped  into  two  cups  filled  with  mercury.  The 
thermo-electric  couple  was,  therefore,  bismuth  and  copper. 
To  generate  current,  junction  ab  was  placed  in  the  hollow 
cylinder  x  of  vessel  A,  while  junction  a'b'  was  placed  in  the 
corresponding  position  in  vessel  B.  The  difference  in  tem- 
perature gave  rise  to  an  electric  current  whenever  the  two 

1  G.  S.  Ohm,  "  Bestimmung  des  Gesetzes,  nach  welcliera  Metalle  die 
Contaktelectrlcitat  leiten,  etc.,"  in  Schweigger''s  Journal  f.  Chemie  u. 
Physik,  Vol.  46,  1826,  p.  144.  This  article  contains,  among  othei 
things,  the  experimental  proof  of  Ohm's  law. 


ELECTRICITY    AND   MAGNETISM 


229 


mercury  cups  were  connected  with  each,  other  by  a  conductor, 
so  as  to  complete  the  circuit.  Ohm  had  a  torsion  balance 
constructed  by  a  mechanic  under  his  direction.  A  magnetic 
needle  was  suspended  from  a  torsion-head  by  a  flattened  wire 
five  inches  long.     When  the  needle  was  deflected  by  the  cur- 


TORSION  BALANCE 


^^ 


X 


BOILING  WATER 


X':^^ 
^ 


X 


ICE     /B 


Fig.  15. 

rent  from  its  position  of  rest  in  the  magnetic  meridian,  it  was 
brought  back  to  its  original  position  by  torsion.  The  angle 
through  which  the  torsion-head  must  be  deflected  was  meas- 
ured in  centesimal  divisions  of  the  circle.  The  force  tending 
to  deflect  the  needle  from  its  initial  position  was  proportional 
to  this  angle.  Hence  the  strengths  of  currents  could  be  com- 
pared by  measuring  the  angles  through  which  the  torsion-head 
was  turned,  in  each  case,  in  order  to  bring  the  needle  back 
to  zero. 

Ohm  prepared  eight  copper  wires  of  equal  thickness  (|-  of  a 
line)  and,  respectively,  2,  4,  6,  10,  18,  34,  66,  130  inches  long.- 
These  were  inserted  as  part  of  the  electric  circuit,  one  after 
the  other.  For  each,  measurements  were  taken  of  the  strength 
of  current.  On  January  8,  1826,  he  obtained  the  following 
data: 

Number  of  conductor,  1,        2,        3,        4,        5,        6,      7,      8. 

Angle  of  torsion  in  cen-  |  g^gg   ^qq^^  ^TTf,  2381,  190|,  1344,  83i,  481 
tesimal  divisions,        J 


230  A    HISTORY   OF   PHYSICS 

On  the  llth.  and  loth  of  the  month  he  took,  each  day,  two 
more  sets  of  readings.  He  tabulates  his  readings  and  then 
says :  "  The  above  numbers  can  be  represented  very  satis- 
factorily by  the  equation, 

b  +  x 

where  X  designates  the  intensity  of  the  magnetic  effect  of 
the  conductor  whose  length  is  x,  a  and  b  being  constants  de- 
pending on  the  exciting  force  and  the  resistance  of  the  remain- 
ing parts  of  the  circuit."  He  gave  the  quantity  b  the  value 
20J,  and,  for  the  set  of  measurements  given  above,  the  quan- 
tity a  the  value  7285.  These  numbers  reproduce  very  closely 
all  the  angular  numbers  given  above.  Take,  for  instance,  the 
third  conductor,  for  which  x  =  6,  then,  by  computation,  X 
becomes  277.53,  its  measured  value  being  277|.  The  experi- 
ments were  varied  by  selecting  brass  wire  resistances,  and 
again  by  taking  for  the  two  temperatures  of  the  thermo-electric 
couples  those  of  melting  ice  and  of  the  room  (7.5°  C).  By 
this  change  in  the  range  of  temperature  Ohm  secured  a  varia- 
tion in  the  electromotive  force,  which  yielded  a  different  value 
for  a,  but  did  not  affect  b.  In  all  cases  the  above  formula  was 
satisfied.  Thus,  the  new  law  was  established,  for  a  represents 
the  electromotive  force,  b  -{-  x  the  total  resistance  of  the  cir- 
cuit, X  the  strength  of  current.  Ohm  then  established  experi- 
mentally the  formulae  giving  the  strength  of  current  for  the 
cases  when  cells  are  grouped  in  series  and  when  in  multiple 
arc.  These  results  were  published  in  1826.  Ohm  deserves 
great  credit  for  introducing  and  defining  the  accurate  notions 
of  electromotive  force,  strength  of  current,  and  electric  resist- 
ance. 

The  following  year  Ohm  published  a  book,  entitled  Die 
Galvanische  Kette,  mathematisch  bearbeitet  (Berlin,  1827).  It 
contained  a  theoretic  deduction  of  Ohm's  law,  and  became  far 


ELECTRICITY   AND   MAGNETISM  231 

more  widely  known  than  his  article  of  1826,  giving  the  experi- 
mental deduction.  In  fact,  his  experimental  paper  was  so 
little  known  that  the  impression  long  prevailed  and  still  exists 
that  he  based  his  law  on  theory  and  never  established  it 
empirically.  This  misapprehension  accounts,  perhaps,  for 
the  unfavourable  reception  of  Ohm's  conclusions.  Professor 
H.  W.  Dove,  of  Berlin,  says  that  "  In  the  Berlin  Jahrbilcher  filr 
wissenschaftUche  Kritik,  Ohm's  theory  was  named  a  web  of 
naked  fancies,  which  can  never  find  the  semblance  of  support 
from  even  the  most  superficial  observation  of  facts ;  ^  he  who 
looks  on  the  world,'  proceeds  the  writer,  'with  the  eye  of 
reverence  must  turn  aside  from  this  book  as  the  result  of  an 
incurable  delusion,  whose  sole  effort  is  to  detract  from  the 
dignity  of  nature.'  "  ^ 

As  Ohm's  great  ambition  was  to  secure  a  university  pro- 
fessorship, we  may  readily  imagine  how  this  lack  of  apprecia- 
tion affected  him.  In  order  to  write  his  book  of  1827,  he  had 
secured  leave  of  absence  and  had  gone  to  Berlin,  where  the 
library  facilities  were  better  than  at  Cologne.  Not  only  did 
he  fail  to  secure  promotion  by  the  publication  of  this  book, 
but  he  incurred  the  ill-will  of  a  certain  school  official  (who 
was  a  supporter  of  Hegelianism  and,  therefore,  opposed  to 
experimental  research)  and,  in  consequence,  he  resigned  his 
position  in  Cologne. 

For  six  years  Ohm  lived  in  Berlin,  giving  three  mathematical 
lessons  a  week  in  the  Kriegsschule,  at  a  yearly  salary  of  300 
thaler.  In  1833  he  secured  an  appointment  at  the  polytechni- 
cum  in  Nlirnberg.  Gradually  his  electric  researches  called 
forth  respect  and  appreciation.  Poggendorff  and  Pechner  in 
Germany,  Lenz  in  Russia,  Wheatstone  in  England,  Henry  in 
America  expressed  their  admiration  for  his  work.     The  Eoyal 

1  Memorial  of  Joseph  Henry,  1880,  p.  489. 


232  A   HISTORY    OF   PHYSICS 

Society  of  London  in  1841  awarded  him  the  Copley  medal.  In 
1849,  at  the  age  of  sixty-two,  the  ambition  of  his  youth  was 
finally  attained.  He  was  appointed  extraordinary  professor 
at  the  University  of  Munich,  and  in  1852  ordinary  professor. 
He  died  two  years  later. 

Wheatstone,  the  great  admirer  of  Ohm,  perceiving  the  neces- 
sity of  more  accurate  means  of  measuring  resistances,  invented 
what  is   known  as    "  Wheatstone's  bridge."      Charles   Wheat- 
stone  (1802-1875)  was  born  near  Gloucester.     He  became  a 
manufacturer  of   musical   instruments,  but  in  1834  accepted 
the  chair  of  experimental  physics  at  King's  College,  London. 
Later  he  retired  to  private  life,  living  on  the  income  from  his 
inventions,  particularly  that  of  the  telegraph.     He  was  an 
experimentalist  of  extraordinary  skill,  but  disliked  to  speak  in 
public.     "  In  fulfilment  of  the  duties  of  his  ofiice  at  King's 
College  he  delivered  a  course  of  eight  lectures  on  sound  .  .  . 
but  his   habitual   though   unreasonable   distrust   of   his   own 
powers  of  utterance  proved  to  be  an  invincible  obstacle,  and 
he  soon  afterwards  discontinued  his  lectures,  but  retained  the 
professorship  for  many  years.     Although  any  one  would  be 
charmed  by  his  able  and  lucid  exposition  in  private,  yet  his 
attempt  to  repeat  the  same  process  in  public  invariably  proved 
unsatisfactory."  ^    For  this  reason  some  of  his  more  important 
investigations  were  brought  before  the  public  by  Faraday  in 
the  theatre  of  the  Eoyal  Institution. 

It  is  interesting  to  note  that  the  measurement  of  resistance 
has  been  brought  to  perfection  chiefly  by  those  interested  in 
the  development  of  the  telegraph.  Wheatstone  invented  the 
rheostat,  but  this  has  been  superseded  by  the  resistance  box, 
which  was  first  used  by  Werner  Siemens.  The  earlier  methods 
of  measuring  resistance  laboured  under  the  defect  of  depending 

1  Proc.  Boy.  Soc.  of  London,  Vol.  24,  p.  xviii. 


ELECTRICITY   AND   MAGNETISM 


233 


Oil  the  constancy  of  the  batteries  used.  This  source  of  trouble 
was  removed  by  Becquerel,  who  introduced  the  differential 
galvanometer,  and  by  Wheatstone,  who  adopted  a  method 
suggested  by  Hunter  Christie,  and  was  led  to  the  invention  of 
"  Wheatstone's  bridge."  In  1843  Wheatstone  describes  two 
forms,  differing  merely  in  the  arrangement  of  the  wires.^  The 
second  is  shown  in  Fig.  16.  Eesistances  Za  and  aC  are  con- 
structed equal  to  each  other ;  resistances  Zc,  db,  he,  fC  are  also 
equal  to  each  other.  The  battery  is  connected  at  Z  and  C,  the 
galvanometer  at  a  and  5.  The  gap  cd  may  be  bridged  by  the 
resistance  to  be  measured ;  the  gap  ef  by  a  rheostat,  the  resist- 


FiG.  16. 

ance  of  which  is  adjusted  until  the  galvanometer  deflection  is 
reduced  to  zero.  Then  the  required  resistance  equals  the 
known  rheostat  resistance.  Wheatstone's  new  instrument  was 
modified  by  Kirchhoff,  who  introduced  a  platinum  wire  of 
uniform  thickness  with  which  movable  contact  was  made. 
Kirchhoff  gave  it  a  triangular  shape ;  Siemens  adopted  the 
long,  rectangular  form. 

The  galvanometer  was  invented  by  J.  8.  C.  Schweigger 
(1779-1857),  professor  at  Halle,  in  1820,  immediately  after 
Oersted's  experiment  became  known.  Schweigger  increased 
the  effective  action  of  the  current  by  carrying  the  wire  many 
times  round  the  magnetic  needle.     In  1825  Leopoldo  Nohili 

1  Phil.  Trans. ,  Vol.  133,  pp.  303-327  ;  Scientific  Papers  of  Sir  Charles 
Wheatstone,  London,  1879,  p.  127. 


234  A   HISTORY    OF   PHYSICS 

(1784-1835)  of  Florence  used  the  astatic  multiplier,  having 
two  needles  rigidly  connected  with  each  other,  and  with  the 
south  pole  of  the  one  pointing  the  same  way  as  the  north  pole 
of  the  other.  In  1839  Claude  Servais  Mathias  Pouillet  (1790- 
1868),  professor  in  Paris,  invented  the  tangent  and  the  sine 
galvanometers.  Great  improvements  in  the  promptness  and 
delicacy  of  action  of  galvanometers  were  effected  by  Sir 
William  Thomson,  who  devised  mirror  galvanometers  for  sig- 
nalling through  submarine  cables.  In  recent  years  a  galva- 
nometer designed  by  A.  UArsonval  is  meeting  with  great 
favour.  In  principle  it  is  the  same  as  the  "  siphon  recorder  " 
of  Sir  William  Thomson,  employed  in  submarine  telegraphy, 
and  as  the  suspended  coil  galvanometer,  used  as  early  as  1836 
by  Sturgeon.  About  1890  C.  Vernon  Boys  recommended  the 
use  of  quartz  fibres  in  place  of  silk  for  needle  suspension  in 
delicate  experimentation. 

Michael  Faraday  (1791-1867),  the  greatest  experimentalist 
of  the  nineteenth  century  in  the  field  of  electricity  and  mag- 
netism, was  born  at  Newington  in  London,  and  was  the  son  of 
a  blacksmith.  "My  education,"  he  says,  "was  of  the  most 
ordinary  description,  consisting  of  little  more  than  the  rudi- 
ments of  reading,  writing,  and  arithmetic  at  a  common  day- 
school.  My  hours  out  of  school  were  passed  at  home  and  in 
the  streets."  ^  In  1804  he  served  as  errand  boy  at  a  book- 
store and  bookbindery  near  his  home.  The  following  year  he 
became  an  apprentice  to  the  bookbinder.  At  this  time  he 
liked  to  read  scientific  books  which  happened  to  pass  through 
his  hands.  "  I  made  such  simple  experiments  in  chemistry," 
he  says,  "  as  could  be  defrayed  in  their  expense  by  a  few  pence 
per  week,  and  also  constructed  an  electrical  machine."  At  the 
age  of  nineteen  he  sometimes  in  the  evening  attended  lectures 

*  B.  Jones,  Life  and  Letters  of  Faraday ^  1870,  Vol.  I.,  p.  9. 


ELECTRICITY   AND   MAGKETISM  235 

given  by  Mr.  Tatum  on  natural  philosophy,  his  brother  pay- 
ing the  admission  fee  for  him.  In  1812  he  had  the  good 
fortune  to  hear  four  lectures  delivered  at  the  Royal  Institution 
by  Sir  H.  Davy,  the  great  chemist.  About  this  time  Faraday 
went  as  a  journeyman  bookbinder  to  a  Frenchman  in  London. 
His  new  work  was  uncongenial.  "  My  desire,"  he  said  later, 
"  to  escape  from  trade,  which  I  thought  vicious  and  selfish, 
and  to  enter  into  the  service  of  science,  which  I  imagined  made 
its  pursuers  amiable  and  liberal,  induced  me  at  last  to  make 
the  bold  and  simple  step  of  writing  to  Sir  H.  Davy,  expressing 
my  wishes,  and  a  hope  that  if  an  opportunity  came  in  his  way 
he  would  favour  my  views ;  at  the  same  time,  I  sent  the  notes 
I  had  taken  of  his  lectures."  Davy  replied,  ^'I  am  far  from 
displeased  with  the  proof  you  have  given  me  of  your  confi- 
dence. .  .  ."  Faraday  became  Davy's  assistant  at  the  Eoyal 
Institution  in  1813.  In  the  autumn  of  that  year  Davy  and  his 
wife  started  on  a  tour  abroad,  Faraday  going  with  them  as 
amanuensis.  After  being  with  Davy  in  France,  Italy,  Switzer- 
land, he  returned  to  the  Hoyal  Institution  in  1815.  Soon  after 
his  return  he  began  original  researches,  and  published  his  first 
paper  in  1816.  He  also  commenced  to  lecture  before  the 
"  City  Philosophical  Society."  In  a  letter  he  wrote  of  "  the 
glorious  opportunity  I  enjoy  of  improving  in  the  knowledge  of 
chemistry  and  the  sciences  with  Sir  H.  Davy."  In  1821  Farar 
day  married,  and  brought  his  wife  to  his  rooms  at  the  Eoyal 
Institution,  where  they  lived  together  for  forty-six  years.  In 
1824  he  was  elected  member  of  the  Eoyal  Society  at  a  time 
when  Davy  was  its  president.  It  is  sad  to  relate  that  jealousy 
on  the  part  of  Davy  led  him  to  oppose  Faraday's  election. 
Nevertheless,  Faraday  always  spoke  with  respect  and  admira- 
tion for  the  talents  of  the  man  who  had  done  so  much  to  start 
him  in  his  early  scientific  career.  In  1825  Faraday  became 
director  of  the  Eoyal  Institution. 


236  A    HISTORY   OF   PHYSICS 

Oersted's  memorable  experiment  of  1820  was  studied  in 
England  by  WoUaston,  who  in  1821,  in  the  presence  of  Davy 
in  the  laboratory  of  the  E-oyal  Institution,  sought  by  experi- 
ment to  convert  the  deflection  of  the  needle  by  the  current 
into  a  permanent  rotation.  He  also  hoped  to  produce  the 
reciprocal  effect  of  a  current  rotating  around  a  magnet.  His 
experiments  failed.  As  previously  noted,  Faraday  began  to 
study  magnetic  rotations,  and  on  the  morning  of  Christmas 
Day,  1821,  he  showed  his  wife  for  the  first  time  the  revolution 
of  a  .nagnetic  needle  around  an  electric  current.^  Faraday 
was  blamed  for  not  mentioning  Wollaston  in  his  paper  de- 
scribing the  experiments,  but  Faraday  justly  claimed  that  he 
was  in  no  way  indebted  to  Wollaston.^ 

His  next  investigations  were  on  the  liquefaction  of  gases, 
on  vibrating  surfaces,  and  chemical  subjects.  In  1831  came 
the  discovery  of  magneto-electricity  and  induction  currents. 
As  early  as  1824  he  had  argued  that  as  a  voltaic  current 
affects  a  magnet,  so  a  magnet  ought  to  react  upon  an  electric 
current.  But  he  could  obtain  no  experimental  evidence  of 
this  effect.  Again,  he  knew  that  an  electrified  body  acts 
upon  an  unelectrified  body,  that  a  wire  carrying  an  electric 
current  is  electrified.  Could  that  wire  excite  in  other  wires 
a  state  similar  to  its  own  ?  In  1825  he  passed  a  current 
through  one  wire  which  was  lying  close  to  another  wire  con- 
nected with  a  galvanometer,  but  obtained  no  result.  The 
momentary  existence  of  the  phenomena  of  induction  then 
escaped  him.     In  1828  he  again  experimented  without  result.^ 

1  John  Ttndall,  Faraday  as  a  Discoverer^  New  York,  1877,  p.  12.  A 
work  on  Michael  Faraday  by  S.  P.  Thompson  is  to  appear  soon. 

2  Faraday  explains  all  in  his  "  Historical  Statement  respecting  Electro- 
magnetic Rotation,"  Experimental  JResearches,  Vol.  II.,  pp.  159-162. 

3  B.  Jones  op.  cit.,  Vol.  II.,  p.  2.  What  follows  is  taken  from  this 
source. 


ELECTRICITY   AND   MAGNETISM 


237 


Fig.  17. 


But  Faraday  persisted.  In  August,  1831,  he  took  a  ring  of 
soft  iron  (Fig.  17),  and  wound  coils  A  and  B  around  it.  Coil 
B  was  connected  with  a  galvanometer. 
When  coil  A  was  connected  with  a 
battery  of  ten  cells,  the  galvanometer 
needle  oscillated  and  settled  at  last 
in  the  original  position.  On  discon- 
necting the  battery  the  needle  was 
again  disturbed.  Faraday  did  not  at 
once  grasp  the  full  significance  of  this.  On  September  23  he 
says  in  a  letter,  "  I  am  busy  just  now  again  on  electromag- 
netism,  and  think  I  have  got  hold  of  a  good  thing, 
but  can't  say.  It  may  be  a  weed  instead  of  a  fish 
that,  after  all  my  labour,  I  may  at  last  pull  up." 
Next  day  he  took  an  iron  cylinder,  surrounded  by 
a  helix  connected  with  a  galvanometer.  Then  the 
cylinder  was  placed  between  the  ]3oles  of  a  bar 
magnet,  as  in  Fig.  18.  "  Every  time  the  magnetic 
contact  at  JSf  and  S  was  made  or  broken,  there  was 
magnetic  motion  at  the  indicating  helix  [galva- 
nometer] —  the  effect  being,  as  in  former  cases, 
not  permanent,  but  a  mere  momentary  push  or 
pull.  .  .  .  Hence  here  [was]  distinct  conversion 
of  magnetism  into  electricity."  This  experiment 
is  the  converse  of  Oersted's  experiment;  an  elec- 
tric current  was  excited  by  a  magnet. 

On  October  1,  1831,  Faraday  discovered  induced 
electric  currents.  A  helix,  wound  with  insulated 
copper  wire  203  feet  long,  was  connected  with  a 
galvanometer.  Another  coil  of  the  same  length 
and  wound  around  the  same  block  of  wood  was 
joined  to  the  poles  of  a  battery  of  ten  cells.  "A 
sudden  jerk  was  perceived  when  the  battery  communication 


Fig.  18. 


238  A  HISTOKY   OF   PHYSICS 

was  made  and  hroTien,  but  it  was  so  slight  as  to  be  scarcely 
visible.  It  was  one  way  when  made,  the  other  way  when 
broken,  and  the  needle  took  up  its  natural  position  at  inter- 
mediate times."  On  October  17  he  produced  the  same  effects 
by  merely  thrusting  a  permanent  steel  magnet  into  a  coil  of 
wire.  The  unexpected  phenomenon  in  these  experiments  was 
that  the  induced  effect  was  not  continuous;  it  was  instan- 
taneous "and  partook  more  of  the  nature  of  the  electrical 
wave  passed  through  from  the  shock  of  a  common  Leyden 
jar,  than  the  current  from  a  voltaic  battery."^ 

These  epoch-making  results  threw  light  upon  the  mysterious 
experiment  of  Arago,  who  in  1824  had  observed  the  motion 
of  a  magnet  caused  by  rotating  a  copper  disk  in  its  neighbour- 
hood. 

Paraday  then,  for  a  time,  dropped  electromagnetism  and 
entered  upon  the  study  of  electrolysis  and  the  voltaic  cell. 
He  discovered  the  laws  of  electrolysis.  The  amount  of  water 
decomposed  is  proportional  to  the  quantity  of  electricity  pass- 
ing through  the  liquid,  no  matter  what  the  electric  pressure 
or  the  area  of  the  electrodes  or  the  conductivity  of  the  liquid 
may  be.  Thus,  the  amount  of  gas  set  free  is  an  exact  measure 
of  the  quantity  of  electricity  passing  through.  He  next  ascer- 
tained that  equal  quantities  of  electricity  decompose  in  differ- 
ent electrolytes  equivalent  amounts.  In  1834  he  introduced 
the  terms  "anode"  and  "kathode." 

In  1834  William  JenTcin  observed  that,  if  the  wire  which 
surrounds  an  electromagnet  be  used  to  join  the  plates  of  a 
single  cell,  a  shock  is  felt  each  time  contact  is  broken,  provided 
the  ends  of  the  wire  are  grasped  one  in  each  hand.    A.  P.  Mas- 

1  Faraday  describes  his  tests  in  Experimental  Besearches  in  Electricity, 
London,  1839,  Vol.  I.  See  also  Ostwald's  Klass.,  No.  81.  The  order  in 
which  the  experiments  are  described  by  Faraday  is  not  quite  the  order 
of  discovery. 


ELECTRICITY   AND   MAGNETISM  233 

son  in  Paris  had  observed  similar  phenomena.  Unaware  of 
Henry's  researches  on  self-induction,  Faraday  began  in  1834 
to  study  this  action,  and  he  recognized  it  as  one  of  '-  induction 
of  an  electric  current  on  itself  " ;  he  succeeded  in  showing  the 
presence  of  an  "  extra  current."  The  "  extra  current "  at  the 
"  break "  had  the  same  direction  as  the  original  current  and 
strengthened  it;  the  "extra  current"  at  the  "make"  flowed 
in  the  same  direction  and  weakened  the  original  current.  This 
theory  of  the  existence  of  an  "  extra  current "  met  at  first 
with  considerable  opposition,  but  was  finally  verified  by  other 
workers. 

On  contemplating  Earaday's  experiments  on  electromag- 
netism,  Tyndall  writes  enthusiastically  as  follows :  "  I  cannot 
help  thinking  .  .  .  that  this  discovery  of  magneto-electricity  is 
the  greatest  experimental  result  ever  obtained.  It  is  the  Mont 
Blanc  of  Faraday's  own  achievements.  He  always  worked  at 
great  elevations,  but  higher  than  this  he  never  attained." 

The  lofty  heights  which  were  scaled  by  the  bold  English 
explorer  were  at  the  same  time  reached  by  an  American 
explorer,  neither  being  conscious  of  the  other's  efforts  until 
the  summit  was  reached.  In  the  discovery  of  magneto-elec- 
tricity the  name  of  Faraday  must  be  accompanied  by  that  of 
Joseph  Henry. 

Joseph  Henry  (1799-1878)  was  born  at  Albany,  New  York. 
At  the  age  of  fifteen  he  entered  the  shop  of  a  watchmaker  as 
an  apprentice,  although  his  chief  ambition  then  was  to  excel 
as  an  actor  and  dramatic  writer.  Accidentally  he  came  across 
Gregory's  Lectures  on  Experimental  Philosophy,  the  perusal  of 
which  created  a  love  for  science.  He  entered  the  Albany 
Academy  as  a  pupil,  and  in  1826  became  professor  of  mathe- 
matics there.  He  was  appointed  in  1832  professor  of  natural 
philosophy  at  Princeton  College,  and  in  1846  secretary  of  the 
newly   established    Smithsonian    Institution    in   Washington, 


240  A   HISTORY    OF   PHYSICS 

He  first  engaged  in  original  investigation  at  Albany  in  1827. 
Both  as  professor  and  as  secretary  his  time  was  so  largely 
taken  up  with  teaching  or  routine  work,  that  but  little  time 
was  left  for  research.  At  the  Albany  Academy  seven  hours 
of  daily  teaching  and  the  want  of  a  room  which  could  be  used 
for  experimentation  prevented  nearly  all  research,  except 
during  vacation  time  — the  month  of  August.  His  researches 
were  carried  on  in  the  large  hall  of  the  Academy,  and  invari- 
ably came  to  a  stop  with  the  first  of  September,  the  time  when 
the  Academy  reopened.^  Henry  was  the  first  to  undertake 
important  original  electrical  experimentation  in  the  United 
States  since  the  time  of  Franklin. 

Henry's  first  improvements  were  in  the  electromagnet.  We 
must  here  premise  that  in  1820  Arago  and  Ampere  magnetized 
steel  needles  by  placing  them  in  a  helix  carrying  an  electric 
current,  that  in  1825  Sturgeon  described  the  earliest  electro- 
magnet worthy  of  the  name.  William  Sturgeon  (1783-1850), 
the  son  of  an  idle  shoemakeT  in  Lancashire,  was  a  self-taught 
scientist,  the  founder  of  a  monthly  periodical,  the  Annals  oj 
Electricity.^  Sturgeon's  electromagnet  of  1825  could  lift  nine 
pounds,  or  about  twenty  times  its  own  weight.  He  used  soft 
iron  in  place  of  steel,  bent  the  iron  in  form  of  a  horseshoe, 
and  varnished  the  iron  in  order  to  insulate  the  single  layer  of 
naked  copper  wire  wound  around  it  in  a  loose  spiral  of  eighteen 
turns.  The  current  was  obtained  from  a  copper-zinc  cell  of 
small  internal  resistance.  The  fact  that  this  horseshoe  became 
a  strong  magnet  as  soon  as  the  current  started  and  lost  its 
power  the  moment  the  current  stopped  made  it  the  object  of 
general   interest.      Professor  Moll  of  Utrecht  constructed  a 

1  Mary  A.  Henry,  "  A  Study  of  the  Work  of  Faraday  and  Henry," 
Electrical  Engineer.,  Vol.  13,  p.  28. 

-  Consult  a  sketch  of  his  life  in  S.  P.  Thompson,  The  Electromagnet, 
New  York,  1891,  pp.  412-418. 


ELECTRICITY   AND   MAGNETISM  241 

horseslloe  magnet  supporting  154  pounds.  But  the  one  who 
introduced  radical  improvements  was  Henry  at  Albany.  In- 
stead of  varnishing  the  iron,  he  insulated  the  copper  wire 
by  covering  it  with  silk ;  instead  of  a  few  turns  of  wire  about 
the  core,  he  put  on  many  turns.  His  first  magnet,  having  400 
turns,  was  exhibited  in  March,  1829.  A  further  improvement 
consisted  in  winding  the  core  with  several  coils,  the  ends  of 
which  were  left  free.  Thereby  the  battery  current  could  be 
made  to  subdivide,  the  coils  being  arranged  in  parallel.  He 
experimented  on  the  proper  size  of  coil  to  be  used  with  bat- 
teries of  different  kinds,  and  arrived  at  the  highly  important 
conclusion  that  one  may  use  an  "  intensit}^ "  magnet  with  a 
long  single  wire,  receiving  current  from  an  "  intensity  "  bat- 
tery, with  cells  grouped  in  series ;  or  one  may  use  a  "  quantity  " 
magnet  with  many  short  wires,  to  be  excited  by  a  "  quantity  " 
battery  of  a  single  large  pair  of  plates.  The  former  magnet 
was  to  be  preferred  when  the  current  was  carried  over  con- 
siderable distances  from  the  cell  to  the  magnet,  as  in  case  of 
telegraphy.  Henry's  electromagnets  were  capable  of  sustaining 
fifty  times  their  own  weight  under  the  stimulus  of  a  single  cell 
with  plates  hardly  a  hand's  breadth  in  length  and  width.^ 

The  originality  of  these  results  is  the  more  conspicuous 
when  we  remember  that  Henry  was  at  this  time  unacquainted 
with  the  law  discovered  by  Ohm  in  1826.  In  1833  Henry 
asked  Dr.  Bache,  "  Can  you  give  me  any  information  about 
the  theory  of  Ohm  ?  Where  is  it  to  be  found  ?  "  It  was  not 
till  1837,  during  his  visit  to  London,  that  he  became  acquainted 
with  Ohm's  theory.^ 

1  Henry  published  his  results  in  the  Am.  Jour,  of  Sci.,  Vol.  19,  Jan- 
uary, 1831,  pp.  404,  405.  Consult  also  the  "  Scientific  Writings  of  Joseph 
Henry,"  in  Smithsonian  Miscellaneous  Collections,  Vol.  30,  1887,  Part  I., 
p.  37. 

2  Mary  A.  Henry,  op.  cit.^  p.  30. 


242  A    HISTORY   OF   PHYSICS 

In  August,  1829,  while  he  was  testing  the  lifting  power  of 
magnets  with  different  lengths  of  wire,  and  by  means  of  his 
^'intensity"  magnet  and  battery  had  made  the  actual  combi- 
nation which  constitutes  the  electric  telegraph  of  to-day,  he 
noticed  an  unexpected  spark  resulting  from  the  break  of  a 
long  coiled  wire  through  which  the  battery  current  had  been 
passing.  "Nature  .  .  .  had  lifted  her  veil  for  a  moment  to 
lure  him  in  a  different  direction,  and  so  it  happened  that  when 
vacation  came  round  again  in  August,  1830,  he  had  taken  up 
the  investigation  of  this  new  phenomenon."  He  recognized 
its  nature,  and  in  1832  published  it  under  the  heading  "  Elec- 
trical Self-induction  in  a  Long  Helical  Wire."^  Faraday's 
investigation  on  "  extra  current "  was  made  in  1834  and  pub- 
lished in  1835.  The  priority  of  the  discovery  of  self-induction 
plainly  belongs,  therefore,  to  the  American  physicist. 

Henry  asked  himself  the  question,  if  electricity  can  produce 
magnetism,  cannot  magnetism  produce  electricity  ?  He  took 
his  "  quantity  "  magnet  at  the  Albany  Academy,  wound  around 
the  middle  of  its  armature  a  coil  of  thin  copper  wire,  the  ends 
of  which  were  connected  with  a  galvanometer,  forty  feet 
away.  The  armature  was  placed  across  the  ends  of  the  mag- 
net ;  the  plates  of  the  battery  were  dipped  into  the  dilute  acid, 
the  magnet  was  excited,  and  the  needle  of  the  galvanometer 
swerved;  the  answer  came,  magnetism  can  produce  electricity. 
Like  Faraday,  Henry  was  surprised  to  find  only  momentary 
effects,  and  that  the  deflection  of  the  needle  at  "break"  was 
in  the  opposite  direction  to  that  at  "  make."  There  is  almost 
conclusive  evidence  to  show  that  this  experiment  was  per- 
formed in  August,  1830,  or  one  year  before  Faraday  made  his 
first  experiment  on  magneto-electricity.^    Henry  was  enthu- 


1  Am.  Jour.  Set,  Vol.  22,  1832,  p.  408. 

2  Mart  A.  Henry,  op.  cit. ,  pp.  53  et  seq. 


ELECTRICITY   AND   MAGNETISM  243 

siastic,  and  was  getting  ready  for  an  exhaustive  series  of  ex- 
periments in  August,  1831.  He  started  to  make  a  much  larger 
electromagnet,  also  a  great  "reel/'  aiming  to  secure  a  machine 
capable  of  powerful  work  —  he  was  endeavouring  to  make  a 
dynamo.  But  vacation  drew  to  a  close  before  it  was  completed.^ 
He  resumed  work,  not  in  August,  1832,  but  in  June !  And 
why  ?  By  chance  he  had  come  upon  a  paragraph  in  a  periodi- 
cal, stating  that  Faraday  had  shown  that  magnetism  can  pro- 
duce electricity.  Faraday's  experimentation  was  given  only 
in  outline.  Henry  could  not  tell  to  what  extent  he  was  antici- 
pated. He  immediately  went  to  work.  Using  his  old  appara- 
tus, he  repeated  the  experiments  mentioned  in  the  notice,  and 
hastily  prepared  a  paper,  printed  in  the  American  Journal  of 
Science,  July,  1832.  This  paper  contains  tests  made  before  he 
had  heard  of  Faraday's  work,  and  also  tests  made  after  that. 
Faraday  had  published  his  discovery  of  magneto-electricity  in 
1831.  While  it  is  almost  certain  that  Henry's  discovery  ante- 
dated Faraday's,  Henry  was  anticipated  in  the  date  of  publi- 
cation. Hence  the  priority  rightly  belongs  to  Faraday.  In 
1837  Henry  was  in  Great  Britain,  and  became  personally  ac- 
quainted with  England's  great  physicists.  Henry  loved  to 
dwell  on  the  hours  he  spent  in  Faraday's  society.  Faraday  and 
Wheatstone  expressed  great  esteem  for  the  American  physicist. 
At  King's  College,  in  London,  Faraday,  Wheatstone,  Daniell, 
and  Henry  once  tried  to  evolve  the  electric  spark  from  the 
thermopile.  The  Englishmen  attempted  it  and  failed.  Henry, 
calling  in  the  aid  of  a  discovery  he  had  made  of  the  effect  of 
a  long  interpolar  wire  wrapped  around  a  piece  of  soft  iron, 
succeeded.  Faraday  became  as  wild  as  a  boy,  and,  jumping 
up,  shouted,  "  Hurrah  for  the  Yankee  experiment."  ^ 


1  Mary  A.  Henry,  op.  cit,  p.  54. 

2  A  Memorial  of  Joseph  Henry,  Washington,  1880,  p.  506. 


24:4  A   HISTORY   OF   PHYSICS 

Henry  carried  on  original  researches  in  various  departments 
of  physics,  but  of  all  his  investigations  the  most  finished  are 
those  on  induced  currents  of  different  orders,  made  in  the 
summer  of  1838  at  Princeton.  As  we  have  seen,  currents  in- 
duced by  currents  were  observed  by  Faraday.  As  Faraday's 
secondary  current  was  but  momentary,  it  was  by  no  means 
self-evident  that  it  coidd  act  as  a  primary  current  and  itself 
induce  a  current  in  a  third  circuit.  Henry  proved  that  in- 
duced currents  of  higher  order  are  possible.  "It  was  found 
that  with  a  small  battery  a  shock  could  be  given  from  the  cur- 
rent of  the  third  order  to  twenty-five  persons  joining  hands ; 
also  shocks  perceptible  in  the  arms  were  obtained  from  a  cur- 
rent of  the  fifth  order."  ^ 

An  observation  which  has  an  important  bearing  on  recent 
electromagnetic  theories  was  made  by  Henry  in  1842.  He 
showed  that  the  discharge  of  the  Ley  den  jar  did  not  consist 
of  a  single  restoration  of  the  equilibrium,  but  of  a  rapid  suc- 
cession of  Hbrations  back  and  forth,  gradually  diminishing 
to  zero.  That  the  Ley  den  jar  discharge  is  oscillatory  was 
shown  again  in  1847  by  Helmholtz  in  his  paper  "  Ueber  die 
Erhaltung  der  Kraft."  But  both  Helmholtz  and  Henry  were 
anticipated  by  Felix  Savary,  who  drew  this  conclusion  from  an 
experiment  as  early  as  1827.^  In  1853  Sir  William  Thomson, 
unaware  of  the  earlier  researches,  concluded  from  theory  and 
mathematical  deduction  that  the  discharge  must  be  oscillatory. 

Henry's  "quantity"  magnet  and  coiled  armature  of  1830  (?), 
and  Faraday's  ring  (Fig.  17),  used  in  1831,  may  be  looked  upon 
as  the  first  transformers.  Inspired  by  Henry's  researches, 
Charles  Grafton   Page   (1812-1868),  a  native  of    Salem   and 

1  Trans.  Am.  Phil.  Soc,  Vol.  VI.  (N.  S.),  p.  303.  Quite  full  accounts 
of  Henry's  work  on  induced  currents  are  given  in  J.  A.  Fleming,  TTie 
Alternate-current  Transformer.,  Vol.  I. 

«  Memorial  of  Joseph  Henry,  1880,  pp.  255,  396,  448. 


ELECTRICITY   AND    MAGNETISM  245 

graduate  of  Harvard  College,  after  1840  examiner  in  the 
patent  office  in  Washington,  invented  what  is  now  known  as 
the  Ruhmkorff's  Coil.  His  earliest  research  was  published  in 
1836.  In  1838  he  had  constructed  an  induction  coiP  of  a  high 
degree  of  perfection.  The  primary  was  of  thick  copper  wire ; 
the  secondary  of  very  thin  wire.  The  vibrations  of  an  auto- 
matic hammer  made  and  broke  a  mercury  contact.  To  shorten 
the  time  of  contact  at  break,  Page  poured  oil  or  alcohol  over 
the  mercury.  Later  this  device  was  suggested  by  others  and 
is  usually  attributed  to  Foucault.  The  platinum  contact,  in 
place  of  the  mercurial  break,  was  first  suggested  in  Germany 
in  1839  by  J.  P.  Wagner  and  by  Neef.  Of  Page's  coil  Uppen- 
born  says :  "  The  effects  which  Page  produced  by  means  of 
this  instrument  were  much  more  intense  than  those  produced 
by  Euhmkorff  with  his,  as  Page  succeeded  with  only  a  single 
Grove  element  in  inducing  in  the  second  circuit  such  a  high 
electromotive  force  as  produced  sparks  4i  inches  in  length 
through  a  vacuum  tube  —  a  result  which  Euhmkorff,  although 
his  invention  created  such  a  great  and  well-deserved  attention, 
did  not  attain."  In  the  year  1850  Page  produced  a  coil  yield- 
ing sparks  through  the  air,  eight  inches  in  length.  Says 
Uppenborn,  "  All  things  considered,  it  is  not  a  little  surprising 
that  while  the  invention  of  the  Ruhmkorff's  coil  was  still  in 
its  infancy,  the  wonderful  output  of  Page's  apparatus  was 
still,  even  in  the  year  1851,  quite  unknown  in  Europe."  Evi- 
dently the  coil  should  have  been  named  after  Page  and  not 
after  Euhmkorff. 

Heinrich  Daniel  Ruhmhorff  or  Rulimkorff  (1803-1877)  was 
born  at   Hanover  in  Germany.     In   1819  he  went   to  Paris 

1  Described  in  Am.  Jour.  Sci.,  Vol.  35,  18-39,  p.  259.  A  drawing  of 
the  coil  is  given  there,  also  in  Bedell,  Principles  of  the  Transformer^ 
1896,  p.  291 ;  in  Fleming,  Alternate-current  Transformer.,  Vol.  IL,  1892, 
p.  26;  and  in  F.  Uppenbokn,  History  of  the  Transformer.,  1889,  p.  7. 


246  A   HISTORY    OF   PHYSICS 

where,  later,  he  started  a  manufactory  of  physical  apparatus. 
A  long  series  of  experiments  resulted  in  the  appearance  in 
1851  of  the  famous  "  Euhmkorff  coil."  It  gave  sparks  in  air 
two  inches  in  length.  In  1858  one  of  his  coils  received  the 
first  prize  of  50,000  francs  at  the  French  Exposition  of  Elec- 
trical Apparatus.^  Jamin  says  that  E-uhmkorff  died  almost  a 
poor  man,  because  he  had  spent  all  his  earnings  in  behalf  of 
science  and  in  works  of  benevolence.^ 

The  E;uhmkorff  coil  is  a  transformer  of  the  ^'  open  magnetic 
circuit"  type,  Avhile  the  commercial  transformer  of  our  day, 
like  Faraday's  original  ring  (Fig.  17),  has  a  "  closed  magnetic 
circuit";  that  is,  in  the  latter  the  magnetic  lines  of  force 
nowhere  pass  through  air,  but  follow  the  easier  iron  path 
throughout.  The  transformer  or  converter  used  in  systems 
of  electric  lighting  or  long-distance  power  transmission,  has 
been  developed  by  Cromwell  Fleetwood  Yarley,  Paul  Jabloch- 
koff,  C.  W.  Harrison,  C.  T.  and  E.  B.  Bright,  S.  Z.  de  Ferranti, 
Carl  Zipernowsky,  Max  Deri,  Otto  Titus  Blathy,  Gaulard  and 
Gibbs,  William  Stanley,  and  others.  Thus,  the  theoretical 
researches  of  Faraday  and  Henry,  carried  on  out  of  pure  love 
for  science,  have  become  the  foundation  of  one  of  the  most 
extensive  commercial  developments  of  modern  times,  and  are 
contributing  vastly  toward  the  progress  of  civilization  and  the 
comfort  of  mankind. 

1  It  has  been  claimed  by  some  that  this  prize  should  have  been  awarded 
to  Edward  Sarmiel  Bitchie  (1814-1895),  the  American  philosophical  instru- 
ment maker,  who  improved  on  Euhmkorff' s  instrument  of  1851  by  dividing 
the  secondary  coil  into  sections  for  the  purpose  of  better  insulation.  This 
division  had  been  previously  suggested  by  Poggendorff.  One  of  Ritchie's 
instruments  was  exhibited  in  England  in  1857.  It  is  alleged  that  Euhm- 
korff secured  one  of  these  and,  copying  it  successfully,  captured  the  grand 
prize.  See  A.  E.  Dolbear  in  Proc.  Am.  Acad,  of  Arts  and  Sciences, 
N.  S.,  Vol.  23,  1895-1896,  p.  359. 

2  Nature,  Yol.  17,  1877,  p.  169. 


ELECTRICITY   AND    MAGNETISM  247 

Eeturning  to  Faraday  at  the  Koyal  Institution  in  London, 
we  find  him  soon  after  1835  working  on  electrostatic  induction. 
Coulomb  and  others  had  assumed  the  theory  of  "action  at  a 
distance  '^ ;  electric  charges  were  supposed  to  attract  and  repel 
each  other  at  a  distance  without  being  affected  in  any  way  by 
the  intervening  medium.  Faraday  had  an  idea  that  this  view 
was  erroneous,  that  electric  attraction  and  repulsion  are  propa- 
gated by  means  of  molecular  action  among  the  contiguous 
particles  of  the  insulating  medium,  which  thereby  participates 
in  the  propagation  of  the  electric  forces.  Hence  Faraday 
termed  such  mediums  "  dielectrics."  Faraday  satisfied  himself 
by  experiments  that  induction  does  not  always  take  place  in 
straight  lines,  as  the  theory  of  an  action  at  a  distance  without 
the  aid  of  an  intervening  medium  would  lead  us  to  believe ;  on 
the  contrary,  induction  takes  place  along  curved  lines,  and 
by  the  action  of  contiguous  particles.  These  curved  lines  he 
termed  "lines  of  force."  His  experiments  showed  that  the 
intensity  of  the  electric  force  between  two  charged  bodies 
varies  with  the  nature  of  the  insulating  medium.  He  was 
thus  led  to  the  capital  discovery  of  what  is  now  termed 
"  specific  inductive  capacity."  Henry  Cavendish  had  arrived 
at  the  same  results  long  ago,  but  had  allowed  these  pearls  of 
scientific  truth  to  be  hidden  away.  Faraday's  apparatus,  by 
which  he  compared  specific  inductive  capacities,  was  in  prin- 
ciple a  Leyden  jar  in  which  the  dielectric  could  be  changed. 
It  consisted  of  two  concentric  spheres.  The  hollow  space 
between  their  surfaces  could  be  filled  with  any  desired 
material.  Taking  air  as  the  standard  dielectric,  he  found  the 
electric  attraction  or  repulsion  for  sulphur  2.26  times  greater ; 
for  shellac,  2.0  times ;  for  glass,  at  least  1.76  times  greater. 
Faraday's  experiments  were  published  in  1837 ;  since  1870 
large  additions  to  our  knowledge  of  this  subject  have  been 
made,  but  owing  to  electric  absorption  the  values  assigned  by 


248  A   HISTORY    OF   PHYSICS 

different  observers  for  the  specific  inductive  capacity  of  various 
substances  show  a  most  perplexing  disagreement. 

In  these  researches  Faraday  created  a  symbolism  which  has 
since  been  universally  adopted  in  teaching  physics.  We  refer 
to  his  ^'  lines  of  force."  He  used  this  term  for  the  first  time 
in  1831  in  connection  with  the  lines  exhibited  by  iron  filings, 
but  the  concept  of  "  lines  of  force  "  was  held  by  others  before 
him ;  for  instance,  by  T.  J.  Seebeck.^  In  Faraday's  reasoning 
"lines  of  force"  took  the  place  of  mathematical  analysis,  a 
knowledge  of  which  he  had  had  no  opportunity  to  acquire. 
Being  debarred  from  following  the  course  of  thought  which 
had  led  to  the  achievements  of  the  French  mathematical 
physicists,  Poisson  and  Ampere,  he  invoked  the  aid  of  "  lines 
of  force,"  which,  in  his  mind's  eye,  he  saw  as  distinctly  as  the 
solid  bodies  from  which  they  emanated.^  In  recent  years 
Faraday's  ingenious  symbolism  has  found  its  way  not  only 
into  the  technique,  but  also  into  elementary  instruction.  Even 
in  Germany,  where  the  theory  of  action  at  a  distance  in  elec- 
tricity and  magnetism  has  had  the  strongest  hold,  Faraday's 
notions  are  finding  acceptance;  for  now  Hertz  has  experi- 
mentally demonstrated  the  correctness  of  the  fundamental 
hypothesis  in  Maxwell's  developments  of  Faraday's  theory  of 
the  dielectric.^ 

Faraday  was  led  by  speculation  to  the  belief  that  there 
existed  some  direct  relation  between  light  and  electricity  or 
magnetism.  Many  experimental  attempts  to  prove  this 
yielded  purely  negative  results,  but  in  1845  his  strong  convic- 
tion was  finally  supported  by  actual  experiment.  "  I  have  at 
last  succeeded  in  illuminating  a  magnetic  curve  or   line  of 

1  See  Seebeck's  paper  in  OstwaWs  Klass.,  No.  63. 

2  See  Clerk  Maxwell,  "Action  at  a  Distance,"  Nature^  Vol.  7, 
1872-1873,  p.  342. 

3  See  A.  ScHiJLKE  in  Zeitschr.  f.  Math.  Unterricht^  Vol.  25,  p.  403. 


ELECTRICITY    AND   MAGNETISM  249 

force,  and  in  magnetizing  a  ray  of  light."  ^  Earaday  caused  a 
polarized  beam  to  pass  through  a  piece  of  "  heavy  glass/'  lying 
in  a  strong  magnetic  field,  due  to  a  large  electromagnet.  By 
means  of  a  Nicol  prism,  it  was  found  that  the  wave  of  light 
was  twisted  round  by  the  action  of  the  magnet  so  that  its 
vibrations  were  executed  in  a  different  plane.  He  says  :  '•  Not 
only  heavy  glass,  but  solids  and  liquids,  acids  and  alkalies, 
oils,  water,  alcohol,  ether,  all  possess  this  power."  Comment- 
ing on  this  relation  between  light  and  magnetism,  Whewell 
wrote  Faraday:  "I  cannot  help  believing  that  it  is  another 
great  stride  up  the  ladder  of  generalization,  on  which  you  have 
been  climbing  so  high  and  standing  so  firm." 

Earaday's  powerful  magnets  and  heavy  glass  led  him  to 
the  verification  of  another  of  his  prophecies.  That  magnetic 
properties  should  be  confined  to  iron  and  nickel  appeared  to 
him  too  extraordinary  to  be  probable.  Knowing  that  the  mag- 
netic strength  of  iron  lessens  at  very  high  temperatures,  he 
suspected  that  other  metals  might  show  magnetism  at  lower 
temperatures.  As  early  as  1836  he  experimented  on  metals 
cooled  to  —  50°  C,  but  without  results.  In  1839  he  repeated 
these  experiments  at  —  80°  C,  again  without  result.  In  1845 
he  added  cobalt  to  the  list  of  magnetic  substances.  In  1846, 
at  last,  he  published  the  general  result.  On  November  4, 1845, 
he  suspended  by  silk  a  bar  of  heavy  glass  between  the  poles  of 
his  new  electromagnet.  When  the  magnet  was  excited  the 
heavy  glass  was  repelled  from  the  poles  so  as  to  assume  an 
equatorial  position.  Earaday  experimented  with  other  sub- 
stances and  found  that  all  liquids  and  solids  were  attracted  or 
repelled,  provided  that  sufficient  magnetic  power  was  used. 
Sulphur,  india-rubber,    asbestos,  tissue   of   the   human  body, 


1  B.  Jones,  op.  cit..  Vol.  II.,  p.  195.     See  also  Experimental  Besearches^ 
19th  Series. 


250  A   HISTORY   OF   PHYSICS 

were  repelled  —  were  shown  to  be  diamagnetic.  Says  Fara- 
day, "If  a  man  could  be  in  the  magnetic  field,  like  Moham- 
med's coffin,  he  would  turn  until  across  the  magnetic  line."  ^ 
Diamagnetic  phenomena  had  been  observed  before,  but  the  ex- 
periments were  not  known  to  Faraday.  Brugmans,  A.  C. 
Becquerel,  Le  Baillif,  Saigey,  and  Seebeck  had  indicated  the 
existence  of  repulsive  force  exercised  by  a  magnet  on  two  or 
three  substances.  Wheatstone  called  Faraday's  attention  to 
BecquerePs  research  on  the  magnetic  condition  of  matter,  and 
Faraday  replied,  "  It  is  astonishing  to  think  how  he  could 
have  been  so  near  the  discovery  of  the  great  principle  and 
fact,  and  yet  so  entirely  miss  them  both,  and  fall  back  into  old 
and  preconceived  notions."  ^ 

1  In  1853  the  London  public  was  greatly  excited  over  the  "  tkble-turn- 
ing"  of  three  skilful  performers.  Without  due  inquiry,  the  effects  were 
referred  to  electricity,  to  magnetism,  or  to  some  unrecognized  physical 
power  able  to  affect  inanimate  bodies.  Faraday  looked  into  the  matter 
and  wrote  in  part  as  follows  :  "I  have  not  been  at  work  except  in  turning 
the  tables  upon  the  table-turners,  nor  should  I  have  done  that,  but  that 
so  many  inquiries  poured  in  upon  me,  that  I  thought  it  better  to  stop  the 
inpouring  flood  by  letting  all  know  at  once  what  my  views  and  thoughts 
were.  What  a  weak,  credulous,  incredulous,  unbelieving,  superstitious, 
bold,  frightened,  what  a  ridiculous  world  ours  is,  as  far  as  concerns  the 
mind  of  man."  Faraday  complains  of  the  great  body  of  men  who  refer 
the  results  "  to  some  unrecognized  physical  force,  without  inquiring 
whether  the  known  forces  are  not  sufficient,  or  who  even  refer  them  to 
diabolical  or  supernatural  agency  rather  than  suspend  judgment  or 
acknowledge  to  themselves  that  they  are  not  learned  enough  in  these 
matters  to  decide  on  the  nature  of  the  action.  /  thi7ik  the  system  of 
education  that  could  leave  the  mental  condition  of  the  public  body  in  the 
state  in  which  this  subject  has  found  it  must  have  been  greatly  deficient  in 
some  very  important  principle.''''  — B.  Jones,  op.  cit.,  Vol.  II.,  pp.  300-302. 

2 In  coining  the  words  "diamagnetic"  and  "paramagnetic,"  Faraday 
consulted  Whewell,  who  wrote  in  1850  in  a  letter  as  follows  :  "  I  am  always 
glad  to  hear  of  your  wanting  new  words,  because  the  want  shows  that  you 
are  pursuing  new  thoughts.  .  .  .  The  purists  would  certainly  object  to  the 
opposition,  or  coordination,  of  ' terromagnetic '  and  'diamagnetic'  .  .  . 


ELECTRICITY    AND   MAGNETISM  251 

Beginning  about  the  time  of  Ampere,  several  new  electric 
theories  came  to  be  advanced.-^  The  early  theories  neglected 
the  action  of  the  dielectric,  but  assumed  the  existence  of  one 
or  two  electric  fluids  and  took  no  account  of  the  principle  of  the 
conservation  of  energy.  The  recognition  by  Faraday  of  the 
influence  of  the  dielectric  medium  "  is,  perhaps,  the  most  im- 
portant step  that  has  ever  been  made  in  the  theory  of  elec- 
tricity." We  have  seen  that  he  was  led  to  this  by  his  desire 
to  get  rid,  as  far  as  possible,  of  the  idea  of  action  at  a  distance, 
which  was  so  prevalent  in  his  time,  but  to  which  his  researches 
have  given  the  death-blow.  Faraday's  ideas  were  expressed 
in  mathematical  language  and  were  more  fully  developed,  so 
as  to  culminate  in  the  electromagnetic  theory  of  light,  by  the 
genius  of  Maxwell. 

James  Clerk  Maxwell  (1831-1879)  was  born  in  Edinburgh, 
enjoyed  good  opportunities  for  early  development,  and  soon 
displayed  power  for  mathematical  and  physical  research.  At 
the  age  of  fifteen  he  published  a  paper  on  oval  curves.  He 
attended  meetings  of  the  Royal  Society  of  Edinburgh.  In 
1847  he  met  William  ISTicol,  the  inventor  of  the  polarizing 
prism,  and  became  interested  in  the  phenomena  of  polarized 
light.     Professor  Campbell "  says  that,  to  keep  their  education 

Hence  it  would  appear  that  the  two  classes  of  magnetic  bodies  are  those 
which  place  their  length  parallel  or  according  to  the  terrestrial  magnetic 
lines,  and  those  which  place  their  length  transverse  to  such  lines.  Keep- 
ing the  preposition  dia  for  the  latter,  the  preposition  para  or  ana  might 
be  used  for  the  former ;  perhaps  para  would  be  best,  as  the  word  '  paral- 
lel,' in  which  it  is  involved,  would  be  a  technical  memory  for  it."  See 
I.  ToDHUNTEK,  William  Wlieioell,  London,  1876,  Vol.  II.,  p.  363. 

1  Consult  J.  J.  Thomson,  "Report  on  Electrical  Theories,"  Beport  of 
the  Brit.  Association,  1885,  pp.  97-155  ;  Helmholtz,  "On  Later  Views  of 
the  Connection  of  Electricity  and  Magnetism,"  Smithsonian  Beport,  1873. 

2  L.  Campbell  and  W.  Garnett,  Life  of  James  Clerk  Maxwell^  Lon- 
don, 1882,  p.  85 ;  we  are  using  also  K.  T.  Glazebrook,  James  Clerk 
Maxwell  and  Modern  Physics,  New  York,  1896. 


252  A   HISTORY   OF   PHYSICS 

at  the  Edinburgh  Academy  "  abreast  of  the  requirements  oi 
the  day,"  etc.,  it  was  thought  desirable  that  they  should  have 
lessons  in  "Physical  Science."  So  one  of  the  classical  masters 
gave  them  out  of  a  text-book.  The  only  thing  I  distinctly 
remember  about  these  hours  is  that  Maxwell  and  P.  G.  Tait 
seemed  to  know  much  more  about  the  subject  than  our  teacher 
did.  In  the  fall  of  1847  Maxwell  entered  the  University  of 
Edinburgh,  learning  mathematics  from  Kelland,  physics  from 
J.  D.  Forbes,  and  logic  from  Sir  Wm.  Hamilton.  Forbes 
gave  him  free  use  of  the  class  apparatus  for  original  experi- 
ments, and  he  worked  without  any  assistance  or  supervision 
with  physical  and  chemical  apparatus,  and  devoured  all  sorts 
of  scientific  w^orks  in  the  library.  In  1850  Maxwell  entered 
the  University  of  Cambridge,  where  he  obtained  the  position 
of  second  wrangler.  At  this  time,  and  later.  Maxwell  was 
fond  of  writing  quaint  verses  which  he  brought  round  to  his 
friends,  "  with  a  sly  chuckle  at  the  humour,  which,  though  his 
own,  no  one  enjoyed  more  than  himself."  ^  Maxwell  became 
professor  of  physics  at  Marischal  College,  Aberdeen,  in  1856 ; 
at  King's  College,  London,  in  1860 ;  at  Cambridge  University, 
in  1871. 

In  papers  on  "  Physical  Lines  of  Force,"  published  in  1861 
and  1862,  and  in  later  papers,  he  translated  Faraday's  theories 
into  the  language  of  mathematics,  and  developed  the  theory  ac- 
cording to  which  the  energy  of  the  electromagnetic  field  resides. 
in  the  dielectric  as  well  as  in  the  conductors.  Faraday  had  said 
that  "  induction  appears  to  consist  in  a  certain  polarized  state 
of  the  particles  into  which  they  are  thrown  by  the  electrified 
body  sustaining  the  action,  the  particles  assuming  positive  and 
negative  points  or  parts.  .  .  .    This  state  must  be  a  forced  one, 

1  Read  verses  in  L.  Campbell  and  W.  Garnett,  op.  cit.,  pp.  577-651, 
particularly  his  parody  of  Tjmdall's  Belfast  Address. 


ELECTRICITY    AND   MAGNETISM  253 

for  it  is  originated  and  sustained  only  by  a  force,  and  sinks 
to  the  normal  or  quiescent  state  when  that  force  is  removed." 
Maxwell  changed'  Faraday's  nomenclature ;  instead  of  the 
polarization  of  the  dielectric,  he  speaks  of  the  change  as  con- 
sisting of  an  "  electric  displacement."  He  looked  upon  the 
action  in  the  dielectric  as  analogous  to  that  of  an  elastic  solid 
which  springs  back  to  its  original  position  when  the  external 
force  is  removed.  The  change  in  electric  displacement  is  an 
electric  current,  called  a  "displacement  current,"  to  distinguish 
it  from  a  current  in  conductors,  designated  as  "  conduction  cur- 
rent." (Hertz  has  proved  the  existence  of  these  "  displacement 
currents  "  by  experiments  which  are  quite  free  from  objec- 
tion.) In  a  medium  supposed  to  be  subject  to  such  electric 
displacement,  waves  of  periodic  displacement  could  be  set  up. 
The  velocity  of  such  a  wave  was  very  nearly  equal  to  that 
of  light.  Hence,  "the  elasticity  of  the  magnetic  medium  in 
air  is  the  same  as  that  of  the  luminiferous  medium  if  these 
two  coexistent,  coextensive,  and  equally  elastic  media  are  not 
rather  one  medium."  That  electromagnetic  phenomena  and 
the  phenomena  called  light  have  their  seat  in  the  same  medium, 
and  are,  in  fact,  identical  in  nature,  is  the  theory  elaborated 
by  Maxwell  in  his  great  Treatise  on  Electricity  and  Magnetism^ 
published  in  1873.  AVhile  this  theory  did  not  contradict  any 
observed  facts,  Maxwell  himself  had  only  few  and  indecisive 

1  This  epoch-making  book  has  always  been  found  difficult  of  compre- 
hension. Poincard  writes  this:  "A  French  savant^  one  of  those  who 
have  most  completely  fathomed  Maxwell's  meaning,  said  to  me,  '  I  under- 
stand everything  in  the  book  except  what  is  meant  by  a  body  charged 
with  electricity.'"  Hertz  expresses  himself  as  follows:  "Many  a  man 
has  thrown  himself  with  zeal  into  the  study  of  Maxwell's  work,  and, 
even  when  he  has  not  stumbled  upon  unwonted  mathematical  difficulties, 
has  nevertheless  been  compelled  to  abandon  the  hope  of  forming  for  him- 
self an  altogether  consistent  conception  of  Maxwell's  ideas.  I  have  fared 
no  better  myself." — Electric  Waves^  trans,  by  D.  E.  Jones,  p.  20. 


254  A  HISTORY   OF   PHYSICS 

criteria  in  support  of  it,  but  Ms  great  prophecy  was  expert 
mentally  confirmed  by  the  illustrious  Hertz. 

Heinrich  Rudolf  Hertz^  (1857-1894)  was  born  at  Hamburg. 
After  leaving  the  gymnasium,  he  fitted  himself  for  civil  engi- 
neering. At  the  age  of  twenty,  he  came  to  a  turning-point 
in  his  career;  he  was  converted  from  a  man  of  practice  to 
one  of  learning.  He  went  to  Berlin,  and,  under  Helmholtz, 
advanced  rapidly.  He  became  in  1880  assistant  to  Helm- 
holtz, in  1883  privat-docent  at  Kiel,  in  1885  professor  of 
physics  at  the  Technical  High  School  at  Karlsruhe.  There 
he  performed  his  memorable  experiments  on  electromag- 
netic waves.  In  1889  he  succeeded  Clausius  at  Bonn,  and 
thus,  at  the  age  of  thirty-two,  occupied  a  position  usually 
attained  much  later  in  life.  In  1892  a  chronic  blood-poisoning 
began  to  undermine  his  health,  and  he  died  in  the  prime  of 
life. 

In  1888  Hertz  found  means  of  detecting  the  presence  of 
electromagnetic  waves  arising  from  Ley  den  jar  or  coil  sparks. 
This  was  an  accomplishment  which  Maxwell  had  feared  would 
never  be  realized.  During  the  oscillatory  discharge  of  a  Ley- 
den  jar,  or  of  a  Holtz  machine,  electromagnetic  waves  radiate 
into  space.  Such  a  wave  is  called  "  electromagnetic  "  because 
it  has  two  components  —  an  electric  wave  and  a  magnetic  wave. 
Hertz  was  able  to  observe  each  separately.  If  electromagnetic 
waves  fall  upon  a  reflector  (a  large  sheet  of  tin,  for  in- 
stance), then  they  are  thrown  back,  and  the  interference  of 
the  two  trains  of  waves,  moving  in  opposite  directions,  gives 
rise  to  places  of  least  and  of  maximum  disturbance  (nodes  and 
antinodes).  Hertz's  detector  consisted  simply  of  a  circular 
wire,  the  ends  terminating  in  brass  knobs,  which  were  adjusted 


1  H.  Ebert  in  Electrician  (London),  Vol.  33,  1894,  p.  272.    See  also 
SI  sketch  by  H.  Bonfort,  in  Smithsonian  Eeport,  1894,  p.  719. 


ELECTRICITY   AND   MAGNETISM  255 

at  small  distances  apart.  A  wave  falling  upon  the  wire,  under 
suitable  conditions,  causes  minute  sparks  to  pass  between  the 
knobs.  Hertz  succeeded  in  reflecting,  refracting,  diffracting, 
and  polarizing  these  waves.  "  The  object  of  these  experi- 
ments," says  Hertz,  "  was  to  test  the  fundamental  hypotheses 
of  the  Earad ay-Maxwell  theory,  and  the  result  of  the  experi- 
ments is  to  confirm  the  fundamental  hypotheses  of  the  theory."  ^ 
Electricity  has  thus  annexed  the  entire  territory  of  light  and 
*' radiant  heat." 

After  Hertz  had  published  his  results,  he  learned  that 
English  experimentalists  had  been  working  in  similar  lines. 
He  says :  "  I  may  here  be  permitted  to  record  the  good  work 
done  by  two  English  colleagues  who  at  the  same  time  as 
myself  were  striving  toward  the  same  end.  In  the  same  year 
in  which  I  carried  out  the  above  research.  Professor  Oliver 
Lodge,  in  Liverpool,  investigated  the  theory  of  the  lightning 
conductor,  and  in  connection  with  this  carried  out  a  series  of 
experiments  on  the  discharge  of  small  condensers  which  led 
him  on  to  the  observation  of  oscillations  and  waves  in  wires. 
Inasmuch  as  he  entirely  accepted  Maxwell's  views,  and  eagerly 
strove  to  verify  them,  there  can  scarcely  be  any  doubt  that  if 
I  had  not  anticipated  him  he  would  also  have  succeeded  in 
observing  waves  in  air,  and  thus  also  in  proving  the  propaga- 
tion with  time  of  electric  force.  Professor  Eitzgerald,  in 
Dublin,  had  some  years  before  endeavoured  to  predict,  with  the 
aid  of  theory,  the  possibility  of  such  waves,  and  to  discover 
the  conditions  for  producing  them.    My  own  experiments  were 

1  Hertz's  papers  are  collected  in  a  book,  Electric  Waves,  trans,  by 
D.  E.  Jones,  London,  1893.  A  full  account  of  Hertz's  experiments  is 
given  in  Fleming,  Alternate-current  Transformer,  Vol.  I.,  in  Preston, 
Theory  of  Light.  See  also  0.  J.  Lodge,  "  The  Work  of  Hertz,"  in  Nature, 
Vol.  50,  1894,  pp.  133-139,  160,  161 ;  Poincare,  "On  Maxwell  and  Hertz," 
in  Nature,  Vol.  50,  1894,  pp.  8-11. 


256  A    HISTORY   OF   PHYSICS 

not   influenced   by  the   researches   of   these  physicists,  for  1 
only  knew  of  them  subsequently."  ^ 

Since  the  publication  of  Hertz's  experiments,  several  new 
detectors  of  electromagnetic  radiation  from  Leyden  jar  or  coil 
sparks  have  been  found.  The  frog's  leg,  to  which  we  owe  the 
discovery  of  current  electricity,  has  been  tried,  but  has  given 
poor  results.  Small  Geissler  tubes  have  been  used  in  place 
of  the  minute  air-gap  in  Hertz's  receiver  or  resonator.  But 
the  most  useful  and  delicate  contrivance  is  the  "  coherer,"  the 
invention  of  which  rests  on  observations  made  independently 
by  Edouard  Branly,  of  the  Catholic  Institute  in  Paris,^  and 
Oliver  J.  Lodge,  of  University  College,  Liverpool.^  As  usually 
constructed,  it  consists  of  a  tube  of  filings  (iron  filings  are 
good),  placed  in  circuit  with  a  voltaic  cell  and  a  galvanometer. 
The  filings  offer  a  high  resistance,  but  as  soon  as  an  electric 
wave  reaches  the  coherer,  the  resistance  breaks  down  through 
a  process  of  electric  welding  between  the  filings,  the  battery 
current  increases  and  gives  a  larger  galvanometer  deflection. 
Improvements  on  Hertz's  vibrator,  or  wave-radiator,  have  been 
made  by  Augusto  Right  of  Bologna.  A  recent  patent  on  wire- 
less telegraphy,  which  has  commanded  much  public  attention, 
is  simply  a  modification  of  the  apparatus  evolved  by  Hertz, 
Lodge,  Branly,  and  Eighi."* 

We  have  seen  that  Ampere,  observing  that  solenoids  act 
like  magnets,  proposed  a  theory  of  magnetism  according  to 
which  all  magnets  were  simply  collections  of  currents.  He 
supposed  that  around  every  molecule  a  minute  current  is  flow- 
ing ceaselessly.     As  such  an  assumption  cannot  be  experimen- 

1  Hertz,  Electric  Waves^  trans,  by  D.  E.  Jones,  p.  3. 

2  Comp.  Rend.,  Vol.  Ill,  p.  785 ;  Vol.  112,  p.  90. 

3  Nature,  Vol.  50,  pp.  133-139. 

*  Electrician  (London),  Vol.  39,  1897,  p.  686  ;  consult  also  0.  Lodge, 
"  History  of  the  Coherer  Principle,"  Electrician,  Vol.  40,  1897,  pp.  87-91. 


ELECTEICITY   AND   MAGNETISM  257 

tally  verified,  and  as  it  savours  somewhat  of  the  fantastic,  later 
theorists  have  been  content  to  assume  with  Simeon  Denis 
Poisson  (1781-1840),  that  each  molecule  becomes  magnetized 
when  the  field  begins  to  act,  or  with  Wilhelm  Weher,  that  the 
individual  particles  are  permanently  magnetic.  Weber  made 
no  attempt  to  explain  the  origin  of  this  magnetism.  He 
advanced  the  view  that  in  hard  steel  there  was  some  kind  of 
friction  between  the  molecules,  which  prevented  the  molecules 
of  magnetized  steel  from  turning  back  into  higgledy-piggledy 
positions.  More  recently  J.  A.  Ewing,  of  the  University  of 
Cambridge,  has  somewhat  modified  Weber's  theory  and  shown 
that  a  complete  explanation  of  the  phenomena  can  be  given 
by  merely  considering  the  forces  which  the  magnetic  mole- 
cules necessarily  exert  on  one  another.  He  prepared  groups 
of  little  magnets,  pivoted  like  compass  needles,  so  that  each 
was  free  to  turn,  except  as  each  was  restrained  by  the  presence 
of  the  others.  An  electromagnet,  whose  strength  could  be 
varied  at  will,  was  used  as  the  external  magnetizing  force. 
With  the  aid  of  this  model  Ewing  was  able  to  imitate  the  phe- 
nomena of  the  magnetization  of  iron  —  how,  with  a  weak  mag- 
netizing force,  magnetism  is  acquired  slowly,  then,  as  the 
external  force  increases,  the  iron  is  gaining  magnetism  fast 
for  a  while,  but  is  approaching  a  third  stage  in  which  the 
rate  of  increment  of  magnetism  falls  off  and  the  iron  ap- 
proaches saturation.^  If  now  the  magnetizing  force  is  grad- 
ually diminished,  then  the  model  again  simulates  a  piece  of 
iron ;  at  first  the  reduction  in  the  magnetization  is  slow,  then 
instability  begins  and  the  magnetization  diminishes  rapidl}^. 
When  the  external  force  is  entirely  removed,  a  little  residual 
magnetism  remains.     As  the  magnetizing  force  is  applied  in 

1  Consult  Ewing,  "The  Molecular  Process  in  Magnetic  Induction,'^ 
Nature^  Vol.  44,  1891,  pp.  566-572.  Reprinted  in  Smithsonian  Beport, 
1892,  pp.  255-268. 


258  A   HISTOilY    OF   PHYSICS 

the  opposite  direction,  the  reversal  of  the  polarity  occurs  with 
a  rush.     "  We  thus  find,"  says  Ewing,  "  a  close  imitation  of  all 
the  features  observed  when  iron  or  any  of  the  other  magnetic 
metals  is  carried  through  a  cyclic  magnetizing  process.     The 
effect  of  any  such  process  is  to  form  a  loop  in  the  curve  which 
expresses  the  relation  of  the  magnetism  to  the  magnetizing 
force.     The   changes   of   magnetism   always   lag   behind  the 
changes  of  magnetizing  force.     This  tendency  to  lag  behind 
is   called   magnetic    hysteresis.^^      When   iron   is   magnetized, 
energy  is   given  to  it;    when   it  is  demagnetized,  energy  is 
taken  from  it.     When  the  magnetization  is  cyclically  altered, 
there  is  a  net  loss,  or  rather  a  waste  of  energy  (a  transforma- 
tion into  heat),  the  amount  of  which  is  proportional  to  the 
area  of  the  loop.     This  heating  Ewing  explains  thus  :    "  When 
the  molecule  becomes  unstable  and  tumbles  violently  over,  it 
oscillates  and  sets  its  neighbours  oscillating."     Heat  is  due  to 
these  oscillations.     When  heated,  iron  is  found  to  be  more  per- 
meable to  magnetization,  until  a  stage  is  reached,  at  a  high 
temperature,  when  the  magnetic  quality  vanishes  almost  sud- 
denly.    This   increase   in  permeability  seems   to   be   due   to 
expansion,  so   that   the  molecular  centres  lie   further  apart, 
and  also  to  the  fact  that  the  molecules  are  thrown  into  vibra- 
tion.    Thereby  the  molecules   tumble  more  easily  from  one 
group  arrangement  into  another.     As  to  the  loss  of  magnetic 
property,  Ewing   says:    "It  is   at   least   a   conjecture  worth 
consideration  whether  the   sudden   loss   of  magnetic  quality 
at  a  higher  temperature  is  not  due  to  the  vibrations  becom- 
ing  so  violent   as   to   set   the  molecules  spinning,   when,  of 
course,  their  polarity  would  be  of  no  avail  to  produce  magnet- 
ization." 

The  study  of  the  magnetic  properties  of  iron  and  steel  has 
received  a  powerful  stimulus  from  the  demands  of  the  design- 
ers   of    dynamos,   motors,    and   transformers.      The    accurate 


ELECTRICITY   Al^D   MAGNETISM  259 

measurement  of  the  relation  of  various  magnetizing  forces 
to  the  magnetizations  produced  in  a  given  piece  of  iron  or 
nickel  was  first  undertaken  by  Henry  A.  Eowland.^  Kow- 
land's  were  the  first  experiments  on  this  subject,  in  which 
the  results  were  expressed  in  absolute  measure,  and  the  rea- 
soning is  carried  out  in  the  language  of  Faraday's  theory  of 
lines  of  magnetic  force.  Rowland  pointed  out  that  the  flow 
of  magnetic  lines  of  force  through  a  magnet  admitted  of 
accurate  calculation,  and  that  the  law  "is  similar  to  the 
law  of  Ohm."  The  word  "permeability,"  denoting  the  ratio 
between  the  magnetizing  force  and  the  resulting  magnetization, 
was  proposed  by  Lord  Kelvin. 

A  concept  which  is  finding  wide  application  in  theoretical 
physics  is  that  of  potential.  Its  origin  we  owe  to  the  mathe- 
maticians, Lagrange  and  Laplace,  who  applied  it  to  gravitatiion 
problems.  The  first  to  apply  the  potential  function  to  a  dif- 
ferent class  of  problems  was  George  Green  (1793-1841),  who 
introduced  it  into  the  mathematical  theory  of  electricity  and 
magnetism.  His  paper  of  1828  escaped  the  notice  even  of 
English  mathematicians  until  1846,  when  Lord  Kelvin  had  it 
reprinted.  Meanwhile  all  of  Green's  general  theorems  had 
been  rediscovered  by  Lord  Kelvin,  Michel  Chasles,  J.  C.  F. 
Sturm,  and  Gauss.  The  mathematicians  defined  potential  as 
that  function  whose  differential  coefficient  with  respect  to  an 
axis  of  coordinates  is  equal  to  the  force  acting  along  that  axis. 
When  the  ideas  of  energy  and  work  came  to  occupy  a  more 
central  position  in  the  minds  of  physicists,  the  term  "  potential " 
was  interpreted  as  signifying  work  done  or  energy  acquired. 
For  instance,  "  electric  potential  at  any  point  is  the  work  that 
must  be  expended  upon  a  unit  of  electricity  in  bringing  it 
to  that  point  from   an   infinite  distance."      The   notion  has 

1  Phil.  Mag.  (4),  Vol.  46,  1873,  p.  140. 


260  A   HISTORY   OF   PHYSICS 

been  made  use  of  in  elementary  instruction,  and  has  often 
been  explained  by  its  analogy  to  temperature  or  difference  of 
level. 

After  the  time  of  Halley,  charts  showing  terrestrial  declina- 
tion were  published  by  Mountain  and  Dodson^  Bellin,  and 
John  Churchman  (Philadelphia,  1790 ;  London,  1794).  The 
question  as  to  the  number  of  the  earth's  magnetic  poles  con- 
tinued to  be  agitated.  Christopher  Hansteen  (1784-1873), 
director  of  the  astronomical  observatory  at  Christiania,  in 
1812  attempted  to  answer  the  prize  question  of  the  Eoyal 
Danish  Academy  of  Sciences,  viz.  "Is  it  necessary,  in  order 
to  explain  facts  in  the  earth's  magnetism,  to  suppose  more 
than  one  magnetic  axis  in  the  earth  ?  "  He  held  the  affirma- 
tive view.  Making  terrestrial  magnetism  his  life  study,  he 
endeavoured  to  subject  to  mathematical  analysis  all  observa- 
tions, with  the  view  of  testing  rigorously  Halley's  speculations 
as  to  the  existence  of  four  magnetic  poles  in  the  earth.  From 
secular  changes  in  the  lines  of  equal  declination  he  inferred 
that  there  were  two  northern  magnetic  poles,  moving  obliquely 
toward  the  west,  and  two  southern  poles,  moving  toward  the 
west ;  that  the  shortest  time  in  which  all  the  poles  return  to 
the  same  relative  position  agreed  closely  with  the  period  of 
revolution  in  the  precession  of  the  equinoxes.  "By  the 
liberality  of  the  Norwegian  government  he  was  enabled  to 
go  to  Siberia,  in  company  with  Due  and  Erman,  to  search  for 
the  ideal  point  of  the  Asiatic  pole  of  magnetism.  They 
started  from  Berlin,  April  25,  1828.  .  .  .  Ten  magnetic  ob- 
servatories were  established  in  the  Russian  empire  by  the 
recommendation  of  Humboldt,  and  great  results  were  reached 
by  Gauss,  Sabine,  Lamont,  and  others  from  the  materials  col- 
lected by  Hansteen  and  Erman.  Hansteen  ascertained  beyond 
dispute  the  existence  of  a  magnetic  pole  in  Siberia  supple- 
mentary to  that  in  British  America,  and  also  the  biaxal  char- 


ELECTRICITY    AND    MAGNETISM  261 

acter  of  the  earth's  magnetism."^  The  fact  that  the  earth's 
magnetism  in  the  northern  hemisphere  reaches  a  maximum  in 
two  places,  viz.  iD  the  north  of  Canada  and  in  the  north  of 
Siberia,  proves  conclusively  that  the  earth  is  not  a  single  mag- 
net. But  neither  Hansteen's  theory  nor  that  of  Sir  Edward 
Sabine  (1788-1883)  seem  to  be  in  accordance  with  observations 
of  more  recent  years.  The  cause  of  the  earth's  magnetism 
and  its  secular  changes  continues  to  be  a  mystery.^ 

An  important  step  toward  the  accurate  study  of  terrestrial 
magnetism  w^as  taken  in  Germany  by  Carl  Friedrich  Gauss 
(1777-1855),  who,  in  conjunction  with  Alexander  von  Humboldt 
(1769-1859),  organized  the  German  Magnetic  Union.  Its 
object  was  to  take  continuous  observations  of  the  magnetic 
elements  (dip,  declination,  intensity)  at  fixed  points.  Obser- 
vations were  begun  in  1834  and  were  mostly  concluded  about 
1842.  Gauss  and  Wilhelm  Weber  (1804-1891)  of  Gottingen 
designed  the  instruments  used  in  these  measurements.  Gauss's 
theory  does  not  aim  to  investigate  the  cause  of  terrestrial  mag- 
netism and  its  changes,  but  is  simply  a  mathematical  presenta- 
tion of  the  distribution  of  magnetism  over  the  earth's  surface. 

Speculations  have  frequently  been  indulged  in  as  to  the 
magnetic  and  electric  relationship  between  the  sun  and  the 
earth.     As  yet,  nothing  very  conclusive  has  been  adduced.^ 

In  a  paper  on  terrestrial  magnetism,  read  in  1832,  Gauss 
proposed  a  system  of  absolute  units.  Since  all  forces  may  be 
measured  by  the  motions  they  produce,  only  three  fundamental 
units  are  necessary,  viz.  a  unit  of  length,  of  time,  of  mass. 

1  ProG.  Boy.  Soc.  of  London,  Vol,  24,  1875-1876,  p.  v. 

2  Consult  further  A.  W.  Rucker,  "Recent  Researches  in  Terrestrial 
Magnetism,"  Nature,  Vol.  57,  1897,  pp.  160  et  seq. 

3  Consult  the  "Abstract  of  a  Report  on  Solar  and  Terrestrial  Magnet- 
ism" by  Erank  H,  Bigelow,  Bulletin  No.  21,  U.  S.  Department  oj 
Agriculture,  1898. 


262  A    HISTORY   OF   PHYSICS 

The  advantage  to  be  gained  is  this :  If  all  practical  units  are 
derived  from  these  three,  then  all  results  of  measurement  are 
comparable  with  each  other.  Gauss  took  as  the  unit  of  force 
that  which  gives  to  unit  mass  in  unit  time  a  unit  velocity.  As 
the  unit  of  magnetic  intensity  he  chose  that  quantity  which, 
acting  upon  an  equal  quantity  at  unit  distance,  exerts  unit 
force.  Gauss's  use  of  absolute  units  in  the  measurement  of 
terrestrial  magnetism  led  his  colleague  at  Gottingen,  Wilhelm 
Weber,  to  introduce  absolute  units  in  electricity.  His  first 
papers  on  the  subject  were  published  in  1846,  1852,  1856.  As 
practical  units  of  resistance,  Moritz  Hermayin  Jacohi  at  St. 
Petersburg  recommended  a  copper  wire  of  given  dimensions, 
the  resistance  of  which  Weber  determined  in  absolute  units. 
As  a  copper  resistance  was  found  to  vary  in  time,  Werner 
Siemens  (1816-1892)  of  Berlin,  in  1860,  proposed  as  a  practical 
unit  the  resistance  of  a  mercury  prism  one  metre  long  and  one 
square  millimetre  in  cross-section,  at  0°C.  ("  Siemens's  unit"). 
Weber  determined  this  in  absolute  units.  In  1861  the  British 
Association  and  Eoyal  Society  of  London  appointed  a  com- 
mittee, with  Lord  Kelvin  at  its  head,  to  recommend  a  unit 
("B.  A.  unit").  Weber's  absolute  unit  of  resistance  was  a 
velocity.  The  British  committee  adopted  this  unit  in  principle. 
In  1881,  at  an  international  congress  of  electricians  in  Paris, 
Weber's  absolute  system  was  adhered  to ;  only,  the  centimetre, 
second,  and  gramme  were  selected  as  primitive  units,  in  place 
of  the  millimetre,  second,  and  milligramme,  used  by  Weber  and 
Gauss.  As  the  ohm  the  congress  selected  10^  times  the  velocity 
of  one  centimetre  per  second.  At  this  time  definitions  were 
given  also  to  the  volt,  amph^e,  coulomb,  and  farad,  along  the 
lines   previously    marked   out    by   Weber.^      The    subject   of 

1  RosENBERGER,  III.,  pp.  302,  514-519;  A.  Kiel,  "  Gescbichte  dei 
Absoluten  Masseinheiten,"  Jahresb.  d.  Konigl.  Gymnasiums  zu  Bonn, 
1890. 


ELECTRICITY   AND   MAGNETISM  263 

* 

"dimensional  equations"  was  first  systematically  presented 
by  Clerk  Maxwell. 

The  securing  of  a  convenient,  invariable  resistance,  equal  to 
10^  absolute  units,  has  been  a  difficult  task.  The  B.  A.  unit 
was  a  little  too  small.  The  "  legal  ohm "  was  provisionally 
adopted  in  1883  by  a  committee  appointed  by  the  congress  of 
1881.  It  was  the  resistance  at  0°  C.  of  a  column  of  mercury 
1  square  millimetre  in  cross-section  and  106  centimetres  long. 
Competent  investigators  like  Eayleigh  and  Mascart  contended 
that  this  column  was  a  little  too  short,  but  some  smaller  values 
obtained  by  certain  experimenters  led  to  the  adoption  of  the 
mean  value  106  centimetres.  The  "legal  ohm"  satisfied  no 
one  and  failed  to  become  legal  in  any  country.-^ 

Henry  A.  Rowland,  after  pointing  out  errors  in  some  of 
the  determinations  previously  made,  found  the  length  of  the 
mercury  column  in  question  to  be  106.32  centimetres.  At  the 
meeting  of  the  British  Association  in  1892,  German,  French, 
and  American  physicists  were  invited  to  take  part  in  the  con- 
sideration of  electrical  units.  The  "B.  A.  unit"  and  the 
"legal  ohm"  were  abandoned.  The  ohm  was  defined  as  the 
resistance  offered  by  a  column  of  mercury  at  the  temperature  of 
melting  ice  14.4521  grammes  in  mass,  of  constant  cross-sectional 
area,  and  of  the  length  of  106.3  centimetres.  By  specifying 
the  mass  of  the  mercury,  instead  of  the  cross-section  of  the 
column,  any  error  arising  from  the  uncertainty  as  to  the  exact 
volume  of  a  gramme  of  mercury  at  0°  C.  was  avoided.  A  system 
of  international  units  was  adopted  at  the  congress  held  in 
Chicago  in  1893  during  the  World's  Fair.  The  ohm,  as  defined 
in  1892,  became  the  iyiternational  ohm.  The  other  units  were 
defined,  including  the  joule  as  a  unit  of  work,  the  luatt  as  a 
unit  of  power,  and  the  henry  as  a  unit  of  self-induction. 

1  H.  S.  Carhart  in  Science,  Vol.  21,  1893,  pp.  86,  87. 


264  A   HISTORY   OF    PHYSICS 

The  electric  discharge  through  partial  vacua  was  carefully 
investigated  after  the  middle  of  the  century.  In  1853  A. 
Masson,  of  Paris,  sent  the  discharge  from  a  powerful  E,uhm- 
korff  coil  through  the  Torricellian  vacuum.  J.  P.  Gassiot 
thereupon  constructed  for  experimental  study  tubes  containing 
a  trace  of  different  gases.  A  few  years  later,  Heinrich  Geissler 
(1814-1879),  a  glass-blower  in  Tubingen,  later  proprietor  of 
a  manufactory  of  physical  and  chemical  apparatus  in  Bonn, 
began  to  prepare  such  tubes  with  so  great  skill  that  they 
have  since  been  named  "Geissler  tubes."  This  designation 
was  proposed  by  Plticker,  who  said,  "  I  give  them  this  name 
and  justly  so,  although  the  first  tubes  were  not  prepared  by 
himself."  ^  The  discharges  through  these  tubes  were  of  great 
beauty,  but  hardly  afforded  a  deeper  insight  into  electricity 
or  the  theory  of  gases.  With  the  improvements  in  mercury 
air-pumps  and  the  attainment  of  higher  degrees  of  rarefac- 
tion, the  phenomena  assumed  a  wider  range.  W.  Hittorf,  of 
Miinster,  in  1869  noticed  that  the  dark  space  separating  the 
negative  pole  from  the  negative  glow  increased  in  width,  as 
exhaustion  was  carried  further  and  finally  filled  the  entire 
tube ;  that  the  discharge  from  the  kathode  caused  considerable 
fluorescence  against  the  glass.  More  striking  and  impressive 
were  the  experiments  which  William  Crookes  began  to  publish 
in  1878.  Crookes  was  born  in  London  in  1832,  and  in  1859 
founded  the  Chemical  News.  His  experiments  on  high  vacua 
began  in  1873,  when,  in  course  of  an  investigation  of  the 
atomic  weight  of  thallium,  he  attempted  to  perform  the  deli- 
cate weighings  in  a  vacuum,  in  order  to  avoid  the  effect  of 
the  buoyancy  of  the  air.  When  heated  bodies  were  weighed 
in  his  exhausted  metallic  box,  the  balance  showed  irregularities 
in  action  which  he  could  not  explain  by  currents  of  air  result- 

1  ROSENBERGEK,    III.,  p.  521. 


ELECTKICITY   AND   MAGNETISM  265 

ing  from  differences  in  temperature.  Crookes  undertook  a 
thorough  investigation  of  the  phenomenon,  and  was  led  in 
1875  to  the  invention  of  the  famous  radiometer.  At  first 
Crookes  and  others  inclined  to  the  opinion  that  the  rotation 
of  the  vanes  was  due  to  the  direct  impact  of  ether-waves. 
But  Crookes  succeeded  in  carrying  the  exhaustion  of  the  bulb 
to  such  a  degree  that  the  vanes  no  longer  rotated.  Hence 
Tait,  Dewar,  and  himself  invoked  the  aid  of  the  modern 
kinetic  theory  of  gases,  and  attributed  the  effect  to  the  mole- 
cules of  the  residual  gas.  The  molecules  impinging  upon  the 
heated  black  surface  of  the  vane  rebounded  with  increased 
momentum,  and  by  their  reaction  propelled  the  vanes.  A 
mathematical  investigation  of  this  action,  based  on  the  kinetic 
theory  of  gases,  was  given  by  Clerk  Maxwell.  In  1878 
Crookes  touched  the  line  of  Hittorf's  researches,  which  were 
apparently  unknown  to  him.  The  thickness  of  the  dark  space 
observed  by  him  and  Hittorf  he  took  to  be  the  "  measure  of 
the  mean  free  path  between  successive  collisions  of  the  mole- 
cules of  the  residual  gas."  In  his  highly  exhausted  tubes 
"  the  molecules  of  the  gaseous  residue  are  able  to  dart  across 
the  tube  with  comparatively  few  collisions,  and  radiating  from 
the  pole  with  enormous  velocity,  they  assume  properties  so 
novel  and  so  characteristic  as  entirely  to  justify  the  applica- 
tion of  the  term  borrowed  from  Faraday,  that  of  "E-adiant 
Matter."  By  beautiful  experiments  he  proved  that  "  Eadiant 
Matter"  proceeds  in  straight  lines,  casts  shadows  when  inter- 
cepted by  solid  matter,  is  capable  of  turning  a  small  wheel, 
is  deflected  by  a  magnet  (shown  previously  by  Hittorf  and 
others).  The  state  and  behaviour  of  the  residual  gas  in 
Crookes's  highly  exhausted  tubes  was  such  that  he  thought 
himself  justified  in  calling  this  an  "  ultragaseous  state,"  or  a 
"  fourth  state  "  of  matter,  differing  as  much  from  the  gaseous 
as  does  the  gaseous  from  the  liquid  state.     The  theory  of  the 


266  A   HISTORY    OF   PHYSICS 

"  fourth  state "  has  been  much  criticised,  particularly  by  the 
Germans,  and  is  not  generally  accepted. 

While  in  the  days  of  Gassiot  the  discharge  from  the  anode 
was  the  subject  of  greatest  attention,  that  from  the  kathode 
still  monopolizes  the  interest.  Hertz  found  that  the  "kathode 
rays  "  will  pass  through  metal  foil.  His  assistant,  P.  Lenard, 
prepared  a  vacuum  tube  with  a  small  window  of  aluminium 
foil,  through  which  he  passed  the  "kathode  rays"  out  into 
the  air.  They  still  retained  their  power  of  exciting  phos- 
phorescence, but  could  not  be  made  to  travel  through  air  but 
a  short  distance.  Lenard  held  that  his  rays  were  not  flying 
particles  but  "phenomena  in  the  ether." ^  While  the  discus- 
sion over  the  nature  of  these  mysterious  rays  was  in  progress, 
Wilhelm  Konrad  Rontgen,^  of  Wtirzburg,  in  1895  discovered 
a  new  kind  of  rays  which  at  once  caused  a  sensation  through- 
out the  civilized  world.  He  found  that  a  Crookes  tube  in 
action  emits  a  radiation  which  causes  a  paper  screen  washed 
with  barium-platino-cyanide  to  light  up  brilliantly  or  to  flu- 
oresce. Paper,  wood,  aluminium,  and  a  great  many  other 
substances  which  are  opaque  to  ordinary  rays  were  found 
transparent  to  the  new  radiation.  The  fact  that  animal 
tissues  are  transparent  and  bones  somewhat  opaque,  makes 
it  possible  for  the  skeletons  of  human  beings  to  be  photo- 
graphed, the  resulting  negatives  being  of  the  nature  of  shadow 
pictures.  The  nature  of  the  new  rays  being  unknown,  Rontgen 
called  them  "X-rays,"  but  they  are  usually  and  more  appro- 
priately called  "Eontgen  rays."  They  show  no  perceptible 
refraction,  nor  regular  reflection  and  polarization.  J.  J. 
Thomson  made  an  experiment  which  seemed  to  prove  that 

1  See  Lenard's  papers  in  Electrician  (London),  Vol.  32,  March  23, 
1893  ;  Vol.  33,  1894,  p.  108. 

2 Eontgen,  "On  a  New  Form  of  Radiation,"  Electrician  (London), 
Vol.  36,  1896,  pp.  415-417,  850,  851. 


ELECTRICITY   AND   MAGNETISM  267 

Eontgen  rays  and  kathode  rays  were  different,  inasmuch,  as 
kathode  rays  inside  a  vacuum  had  no  power  of  exciting  the 
photographic  plate.^  He  also  found  that  these  rays  make 
insulators  conduct  and  consequently  are  able  to  discharge  elec- 
trified bodies.  Improved  tubes  —  the  so-called  "focus-tubes" 
—  were  designed  for  radiography.  An  important  discovery 
which  apj)ears  to  be  a  link  towards  establishing  continuity 
between  the  old  and  the  new  forms  of  radiation  was  made  in 
1896,  in  Paris,  by  Henry  Becquerel,  of  the  Conservatoire  des 
Arts  et  Metiers.  He  is  the  son  and  successor  of  Edmond 
Becquerel  and  the  grandson  of  A.  C.  Becquerel.  He  observed 
that  certain  uranium  compounds,  after  exposure  to  sunlight, 
emitted  radiations  which,  like  Eontgen  rays,  could  pass  through 
plates  of  aluminium  or  of  cardboard,  but  which  could  also  be 
refracted  and  polarized.  Allied  to  both  of  these  are  the  rays 
emitted  by  thorium  and  its  compounds,  which  were  discovered 
almost  simultaneously  by  Sklodowska  Curie  and  G.  C.  Schmidt. 
Thorium  rays  can  be  refracted,  but  cannot  be  polarized  by 
transmission  through  tourmaline. 

We  have  two  methods  for  producing  very  high  differences  of 
electric  potential :  one  is  by  induction  coils  like  Euhmkorff's, 
the  other  is  by  electrical  influence  machines.  These  machines 
have  been  evolved  from  the  electrophorus  of  Volta,  through 
the  improvements  due  to  Georg  Christoph  LicJitenberg  (1742- 
1799)  of  Gottingen,  Abraham  Bennet,  Tiberius  Cavallo  (1749- 
1809)  of  London,  William  Nicholson  (1753-1815,  editor  of 
Nicholson's  Journal  of  Natural  Philosophy,  Chemistry,  and  the 
Arts,  London),  Belli,  Varley,  Kelvi^i,  Topler,  Holtz,  Wimshurst, 
and  others. 

The  first  marked  advance  in  the  design  of  these  machines 

1  It  becomes  more  and  more  clear  that  kathode  rays  consist  of  electrified 
atoms  or  ions  in  rapid  progressive  motion,  and  that  Rontgen  rays  are 
waves  or  pulses  in  the  ether. 


268  A    HISTORY   OF   PHYSICS 

was  made  in  1865.  In  that  year  machines  were  brought  out 
by  A.  Topler  of  Dorpat,  later  professor  at  the  Polytechnicum 
in  Dresden,  and  by  W.  Holtz.  The  latter  soon  improved  his 
machine,  while  Topler,  in  1879,  united  the  principles  of  the 
two  machines  into  the  '^  Topler-Holtz  machine."  A  similar 
one  was  constructed  in  1880  by  the  mechanic,  J.  R.  Voss,  of 
Berlin.  The  machine  with  radial  strips  of  tin-foil  and  contact 
brushes  was  described  by  Holtz  in  1881,^  and  again  in  1882 
and  1883  by  James  Wimshurst,^  who  made  his  improvements 
independently  of  Holtz.^ 

Thermo-electricity  was  discovered  in  1821  by  Hiomas  Joliann 
Seebeck  (1770-1831).  He  was  born  in  Eeval  (Esthonia, 
Eussia).  At  the  age  of  seventeen  he  left  his  native  country, 
never  again  to  return.  He  studied  medicine  in  Berlin.  Being 
well  off,  he  was  free  to  devote  himself  to  science.  From  1802 
to  1810  he  lived  in  Jena,  and  had  a  personal  acquaintance  with 
Schelling,  Hegel,  Eitter,  Gothe,  and  other  prominent  men. 
Unfortunately  he  allowed  himself  to  be  completely  dominated 
by  the  erroneous  anti-Kewtonian  views  on  colour,  so  elaborately 
and  confidently  set  forth  by  Gothe  in  his  Farbenlehre.  Being 
elected  a  member  of  the  Berlin  Academy  of  Sciences  in  1818, 
Seebeck  took  up  his  residence  in  that  city.  Oersted's  experi- 
ment induced  him  to  enter  upon  a  long  series  of  electric  inves- 
tigation. With  the  view  of  verifying  certain  speculations 
regarding  the  magnetic  character  of  the  electric  current,  he 
established  an  electric  circuit  consisting  partly  of  copper  and 
partly  of  bismuth.     One  metallic  junction  he  held  in  his  hand. 

1  Uppenborn's  Zeitschr.f.  angewandte  EleJctr.,  1881,  p.  199. 

2  Engineering,  Vol.  35,  1883,  p.  4. 

3  Consult  articles  on  the  theory  of  recent  types  of  machines,  written  by 
Holtz,  Wimshurst,  and  V.  Schaffers,  in  Electrician  (London),  Vol.  35, 
1895,  pp.  382-388.  See  also  John  Gray,  Electrical  Influence  Machines, 
Whittaker  &  Co. 


ELECTRICITY    AND   MAGNETISM  269 

He  satisfied  himself  that  the  resulting  deflection  of  the 
galvanometer  needle  arose  from  the  difference  in  temperature 
of  the  metallic  junctions,  brought  about  by  the  heat  from  his 
hand.  He  found  similar  effects  by  cooling  one  of  the  junc- 
tions ;  the  effects  varied  for  different  metals,  and  were  greater 
for  greater  differences  of  temperature.  He  used  the  expres- 
sion ^Hhermomagnetic''  currents,  and  later  objected  to  the 
term  "  thermo-electric." 

Thirteen  years  after  Seebeck's  discovery,  Jean  Charles 
Athanase  Peltier^  (1785-1845),  a  Parisian  watchmaker,  who 
devoted  the  latter  part  of  his  life  to  scientific  pursuits,  demon- 
strated that,  conversely,  an  electric  current  may  produce  not 
only  heat  but  also  cold.  In  copper-antimony  junctions  he 
found  a  heating  of  10°  where  the  current  went  from  antimony 
to  copper,  and  a  cooling  of  5°  where  it  went  in  the  opposite 
direction.  Greater  differences  were  found  for  bismuth-anti- 
mony joints.  Heinrich  FriedricJi  Emil  Lenz  (1804-1865),  well 
known  for  his  law  of  electromagnetic  induction,  succeeded  in 
freezing  water  by  the  Peltier  effect. 

After  the  principles  of  electromagnetism  were  established 
by  Paraday  and  Henry,  constant  efforts  were  made  in  the  way 
of  practical  application.  The  early  dynamo  machines  laboured 
under  two  defects :  the  magnetic  intensity  was  not  adequate  or 
properly  applied,  and  the  electric  current  generated  was  not 
sufiiciently  steady.  The  concentration  of  the  magnetic  lines 
of  force  in  a  powerful  field  between  the  magnetic  poles  was 
effected  in  1856  by  Werner  Siemens  in  Berlin  through  his 
improved  shuttle  armature,  with  its  coils  of  wire  wound  upon 
a  grooved  iron  core.  Ten  years  later  Henry  Wilde  of  Man- 
chester substituted  electromagnets  for  the  permanent  steel 
magnets    previously    employed.      He     took    three    Siemens 

1  See  "Memoir  of  Peltier"  in  Smithsonian  Beport,  1867,  pp.  158-202. 


270  A   HISTORY   OF   PHYSICS 

machines,  two  of  which  had  electromagnets.  The  machine 
with  steel  magnets  generated  a  current  which  was  used  to 
excite  the  field  magnets  of  the  second  machine ;  the  armature 
current  from  the  second  excited  the  field  magnets  of  the  third. 
The  current  from  the  last  was  used  in  experimentation.  An 
electric  lamp  was  made  to  give  an  intense  light,  which  caused 
great  astonishment  among  the  populace.  When  passed  through 
a  convex  lens,  the  light  ignited  paper.  The  electric  arc  melted 
not  only  iron  wire,  but  a  rod  of  platinum  6  millimetres  thick 
and  61  centimetres  long.  The  arc  was  still  a  novelty  to  people, 
notwithstanding  the  fact  that  sixty-six  years  earlier,  in  1800, 
it  was  noticed  by  Sir  Humphry  Davy,  and  at  a  still  earlier 
date  by  J.  W.  Ritter.  Davy  used  in  his  experiments  a  battery 
of  2000  cells  and  rods  of  charcoal. 

In  1866  Werner  Siemens  demonstrated  by  the  operation  of  a 
new  machine  of  his  own  construction  that  electromagnets  can 
be  used  without  separate  exciters,  and  that  the  field  magnets 
may  be  excited  by  the  current  from  the  armature  of  the 
machine  itself.  This  idea  appears  to  have  been  in  the  air; 
for,  about  the  same  time  and  independently,  it  was  advanced 
by  Murray,  Cromwell  Fleetivood  Varley  (1828-1883),  C.  Wheat- 
stone,  and  others.  In  Siemens's  armature  the  coils  are  wound 
around  a  cylindrical  core.  Another  typical  armature  is  that 
in  which  the  coils  are  wound  upon  a  ring.  This  was  invented 
in  1861  by  Antonio  Pacinotti  of  Florence,  and  again  inde- 
pendently, in  1868,  by  Zenohe  Theophile  Gramme  of  Paris. 
Through  Gramme  this  armature  came  to  be  extensively  used. 
Since  their  day  the  construction  of  dynamos  for  various  pur- 
poses has  been  carried  to  great  perfection.  Machines  of  high 
merit  were  produced  by  the  Siemens  brothers,  Charles  F. 
Brush,  Thomas  A.  Edison,  and  others.^ 

1  For  details  see  S.  P.  Thompson,  "Historical  Notes,"  in  Dynamo 
Electric  Machinery. 


ELECTRICITY   AND   MAGNETISM  271 

The  design  of  practical  dynamos  made  electric  lighting  pos- 
sible. Arc-lighting  was  never  a  success  until  means  were 
thought  out  for  rendering  lights  placed  in  series  automatic 
in  action  and  somewhat  independent  of  each  other.  Such  a 
regulator  was  invented  in  1847  by  W.  E.  Staite ;  later  Werner 
Siemens  and  others  worked  out  designs.  Among  the  patterns 
are  clockwork  lamps,  solenoid  lamps,  and  clutch  lamps. 

For  house  illumination  arc-lights  were  not  well  adapted. 
A  less  brilliant  light  was  needed.  In  the  years  1877-1880 
inventors  arose  to  the  emergency  by  the  production  of  the 
incandescent  lamp.  The  names  associated  with  this  develop- 
ment of  applied  electricity  are  Joseph  Wilson  Swan  and  Lane- 
Fox  in  England,  Hiram  S.  Maxim,  William  Edward  Saioyer, 
Alhon  P.  Man,  and  Thomas  A.  Edison. 

In  early  experiments  platinum  wire  was  tried  as  the  sub- 
stance to  be  heated  to  whiteness  by  the  passage  of  the  electric 
current.  In  1878  Edison  was  thus  engaged,  but  neither  plati- 
num nor  iridium  could  be  kept  from  the  risk  of  fusing.  In 
the  same  year  Sawyer  and  Man  of  New  York  tried  to  prepare 
carbon  fibres  from  vegetable  tissue.  They  endeavoured  to  pre- 
vent combustion  of  the  fibre  by  filling  the  globe  with  nitrogen, 
but  the  process  was  not  successful.  Lane-Eox,  in  1879,  being 
convinced  that  platinum  and  iridium  were  useless  as  bridges 
in  lamps,  used  carbonized  vegetable  fibres.  Swan,  in  February, 
1879,  made  a  public  exhibition  of  a  lamp  with  a  carbon  fila- 
ment in  a  vacuous  bulb.  Swan's  success  led  Edison  to  abandon 
platinum  and  iridium,  and,  in  October,  1879,  he  had  constructed 
a  vacuum  lamp  with  a  filament  of  lampblack  and  tar  car- 
bonized. In  January,  1880,  Swan  prepared  filaments  from 
cotton  twine,  prepared  by  immersion  in  sulphuric  acid  and 
then  carbonized.  Edison  sent  out  explorers  into  South  America 
and  into  the  far  East  in  quest  of  suitable  fibres  for  lamps,  and 
in  1880  employed  a  flat  strip  of  carbonized  bamboo  for  a  fila- 


272  A    HISTORY   OF   PHYSICS 

inent.  Most  of  the  modern  lamps  have  filaments  prepared 
from  parchmentized  cellulose,  afterwards  carbonized.  The 
race  between  the  several  experimenters  was  indeed  close  and 
exciting ;  numerous  lawsuits  over  the  validity  of  patents 
followed  the  commercial  introduction  of  the  new  lamps.^ 

The  discovery  that  the  action  of  the  dynamo  is  simply  the 
converse  of  the  electric  motor,  so  that  the  same  machine  can 
be  used  either  as  dynamo  or  motor,  was  made  by  M.  H.  Jacobi 
in  1850.  The  principle  of  transmitting  power  from  one 
dynamo  as  a  generator  to  another  used  as  a  motor  was  first 
pointed  out  and  demonstrated  at  the  Vienna  Exhibition  in 
1873  by  Eontaine  and  Gramme.  Since  then  great  progress 
has  been  made  in  the  details  of  design  of  motors. 

After  much  experimentation  in  the  United  States  and  else- 
where on  the  design  of  electric  railways,  the  first  electric 
railway  was  put  in  operation  by  the  firm  of  Siemens  and 
Halske  in  1879  at  the  Industrial  Exhibition  in  Berlin.^ 

Up  to  1883  the  progress  made  in  electric  roads  was  mainly 
due  to  Werner  Siemens  in  Germany,  but  at  this  time  sub- 
stantial advances  were  made  in  the  United  States  by  the 
labours  of  C.  J.  Van  Depoele,  Leo  Daft,  F.  J.  Sprague,  and 
others. 

The  first  polyphase  motor  was  exhibited  to  the  Royal  Society 
of  London  in  1879  by  Walter  Baily.  It  was  a  mere  toy  and 
received  no  further  attention.  A  two-phase  motor  was  con- 
structed and  used   by  Galileo  Ferrans  in  his   laboratory  at 


1  For  a  fuller  account  see  F.  L.  Pope,  Evolution  of  the  Electric  Incan- 
descent Lamp,  1889. 

2  In  the  United  States,  at  this  time,  Edison,  Stephen  D.  Fields,  and 
Wellington  Adams  were  experimenting  on  electric  roads,  and  applying 
for  patents.  See  W.  Adams,  "The  Evolution  of  the  Electric  Railway," 
p.  9,  reprinted  from  the  Jour,  of  the  Ass.  of  Eng.  Societies,  of  September 
and  October,  1884. 


ELECTRICITY   AND   MAGNETISM  273 

Turin  in  1885.  He  used  two  independent  alternate  currents 
of  tlie  same  period,  but  differing  in  phase  and  thus  producing 
a  rotary  magnetic  field.  Thinking  that  no  motor  requiring 
more  than  two  w^ires  could  interest  any  one  but  the  theoretical 
physicist,  he  did  not  publish  his  results  till  1888.^  Only  a 
few  months  later,  commercial  motors  based  on  the  same  prin- 
ciples were  brought  out  by  Nikola  Tesla,  then  at  Pittsburg, 
who  had  made  the  discovery  independently.  A  remarkable 
rotary -field  motor,  devised  by  Dolivo  Dohroivolsky,  was  used 
at  the  Frankfort  Exposition  of  1891.  Many  forms  of  such 
motors  have  been  constructed  since  and  are  meeting  with 
extended  application  in  both  Europe  and  America. 

After  the  principles  of  electromagnetism  were  made  known 
through  the  epoch-making  researches  of  Faraday  and  Joseph 
Henry,  telegraphy  seemed  a  comparatively  easy  matter.  So 
many  investigators  busied  themselves  w^ith  this  idea,  and  per- 
formed experiments  which  were  more  or  less  successful,  that 
it  is  difficult  to  assign  the  invention  of  the  telegraph  to  any 
one  individual.  The  transmission  of  signals  by  electromag- 
netic apparatus  was  suggested  by  Ampere  in  1821.  Gauss 
and  Weber  at  Gottingen  in  1833  had  a  crude  telegraphic  line 
between  the  Observatory  and  the  Physical  Cabinet,  a  distance 
of  9000  feet.  Joseph  Henry  at  Albany,  in  1831,  by  the  attrac- 
tion of  an  electromagnet  produced  audible  signals  at  a  dis- 
tance. In  1837  Morse  of  New  York  devised  a  telegraph  in 
which  the  attraction  of  an  armature  produced  dots  and  dashes 
upon  a  moving  strip  of  paper.  Karl  August  Steinheil  of 
Munich  discovered  that  the  earth  may  take  the  place  of  a 
wire  for  the  return  circuit.  The  first  commercial  line  in  the 
United  States  was  erected  between  Washington  and  Baltimore 


1  See  translation   of  paper  in  Electrician   (London),  Vol.   36,  1895, 
p.  281;  see  also  Nature,  Vol.  44,  1891,  p.  617. 


2?4  A   HISTORY   OF  PHYSICS 

through  the  efforts  of  Morse.  Samuel  Finley  Breese  Morse 
(1791-1872)  was  educated  as  an  artist,  and  is  the  founder  of 
the  National  Academy  of  Design  in  New  York.  He  studied 
art  in  the  schools  of  the  Continent.  While  on  his  ocean 
voyage  homeward,  in  1832,  the  first  thought  of  the  telegraph 
suggested  itself  to  him.  He  experimented  for  several  years 
with  some  success.  Finally,  his  assistant,  Dr.  Gale,  applied 
the  principles  discovered  by  Henry  to  render  Morse's  machine 
effective  at  a  distance.^  After  many  discouragements,  Morse 
established,  by  aid  of  the  American  government,  the  telegraphic 
line  between  Washington  and  Baltimore.  On  May  24,  1844, 
the  message  was  sent  from  the  rooms  of  the  United  States 
Supreme  Court,  '•  What  hath  God  wrought !  "  Morsels  appa- 
ratus is  now  the  most  extensively  used  of  all. 

In  England  the  telegraph  was  developed  by  Wheatstone, 
William  Fothergill  Cooke,  and  Hughes.  Methods  of  duplex 
signalling  (two  messages  sent  simultaneously  in  opposite  direc- 
tions) were  devised  by  WiUiehn  Gint  (1803-1883;  professor 
at  Gratz)  in  1853  and  J.  B.  Stearns  of  Boston  in  1870;  of 
diplex  signalling  (two  messages  sent  at  once  in  same  direction 
through  a  wire),  by  Stark  of  Vienna  and  Bosscha  of  Ley  den  in 
1855 ;  of  quadruplex  telegraphy,  by  0.  Heaviside  of  London  in 
1873  and  T.  A.  Edison  of  Newark  in  1874. 

Experimentation  on  submarine  telegraphy  began  as  early 
as  1837.^  After  some  successes  with  shorter  lines,  the  first 
Atlantic  cable  expedition  was  started  in  1857.     One  of  the 


1  See  "  Statement  of  Professor  Henry  in  Relation  to  the  History  of  the 
Electromagnetic  Telegraph,"  Smithsonian  Beport,  1857,  pp.  99-106 ; 
"  Henry  and  the  Telegraph,"  by  William  B.  Taylor,  in  Smithsonian 
Beport,  1878,  pp.  262-360,  containing  much  detailed  information  on  the 
history  of  the  telegraph. 

2Eor  details  consult  W.  E.  Atrton,  "Sixty  Years  of  Submarine 
Telegraphy,"  Electrician  (London),  Vol.  38,  1897,  pp.  545-519. 


ELECTRICITY   AKD   MAGKETISM  275 

questions  debated  some  years  before  was  the  probable  speed 
of  signalling  through  a  cable  2000  miles  long.  Great  vague- 
ness then  existed  as  to  the  way  in  which  electricity  travelled. 
Wheatstone  had  proved  in  1834  with  aid  of  revolving  mirrors 
that  electricity  travelled  with  a  velocity  of  288,000  miles  per 
second;  but  Latimer  Clark,  from  experiments  made  in  the 
presence  of  Airy  and  Faraday  on  800  miles  of  underground 
wire,  came  to  the  conclusion  that  it  took  half  a  second  before 
the  current  appeared  at  the  other  end.  Other  experimenters 
obtained  intermediate  results. 

The  explanation  of  these  discrepancies  was  given  by  a  young 
man,  William  Thomson,  now  Lord  Kelvin,  in  a  correspondence 
with  Sir  Gabriel  Stokes.  This  correspondence  formed  the 
basis  of  Thomson's  very  important  paper,  published  in  the 
Pi-oceedings  of  the  Eoyal  Society  in  1855.  One  of  the  first 
conclusions  theoretically  deduced  by  him  was  that  electricity 
has  no  velocity  at  all.  Just  as  the  time  for  the  flow  of  heat 
through  a  rod  depends  only  on  the  rod,  so  the  time  before  the 
current  begins  to  appear  at  the  other  end  depends  only  on  the 
cable  —  that  is,  upon  the  product  of  its  resistance  and  its  elec- 
trostatic capacity.  The  opinion  of  well-known  engineers  of 
the  time  was  opposed  to  this.  Thomson  also  tried  to  make  it 
plain  that  it  would  take  so  long  for  the  current  to  reach  its 
steady  state  at  the  end  of  an  Atlantic  cable  that,  if  they  ever 
wanted  the  cable  "  to  pay,"  they  must  not  wait  for  the  current, 
but  must  send  messages  with  currents  at  the  very  beginning 
of  their  growth.  Another  important  conclusion  reached  by 
him  was  that  the  retardation  of  signals  was  proportional  to 
the  square  of  the  length.  Thomson  estimated  the  probable 
speed  of  the  proposed  cable  at  three  words  per  minute,  Wer- 
ner Siemens  at  one  word  per  minute,  Sir  Charles  Bright  at 
ten  or  twelve  words  per  minute.  The  results  gave  for  ordi- 
nary recording  instruments  1.8  words  per  minute.     On  August 


276  A    HISTORY   OF   PHYSICS 

5,  1858,  England  and  America  had  the  first  cable  communica 
tion.  The  President  of  the  United  States  sent  a  message  con- 
taining the  prayer,  "May  the  electric  telegraph,  under  the 
blessing  of  Heaven,  prove  to  be  a  bond  of  j)erpetiial  peace  and 
friendship  between  the  kindred  nations."  One  hundred  and 
fifty  words  were  transmitted  in  thirty  hours.  As  time  went 
on  the  signals  grew  weaker,  and  in  a  month  the  Atlantic  cable 
ceased  to  speak.  William  Thomson  calculated  what  would  be 
the  best  proportions  for  the  new  Atlantic  cable,  which  was  suc- 
cessfully laid  in  1866.  He  designed  apparatus  to  be  used  in 
signalling.  The  astatic  reflecting  galvanometer  was  a  much 
improved  form  of  the  mirror  galvanometer  originally  devised 
by  Gauss  and  Weber,  and  employed  on  their  telegraph  line  in 
Gottingen.  Thomson's  galvanometer  raised  the  speed  of  cable 
telegraphy  from  two  or  three  to  twenty-two  or  twenty-five 
words  per  minute.  On  account  of  the  great  fatigue  to  the  eye 
in  following  the  motion  of  a  spot  of  light  in  the  mirror  gal- 
vanometer, it  was  discarded  in  signalling  through  cables,  and 
Thomson's  "  siphon  recorder "  adopted.  The  researches  of 
Thomson,  as  continued  by  Cromwell  E.  Varley,  showed  that 
the  speed  can  be  increased  still  further  by  sending  a  positive 
current,  and  then  a  negative  one  for  a  short  time. 

The  earliest  record  of  a  theoretical  telephone  was  contained 
in  Du  Moncel's  Expose  des  Applications,  Paris,  1854,  when 
Charles  Bourseul,  a  French  telegraphist,  conceived  a  plan  of 
transmitting  speech  by  electricity.  The  author  says,  "  Sup- 
pose a  man  speaks  near  a  movable  disk  sufficiently  flexible  tc 
lose  none  of  the  vibrations  of  the  voice ;  that  this  disk  alter- 
nately makes  and  breaks  the  currents  from  a  battery,  you  may 
have  at  a  distance  another  disk  which  will  simultaneously  exe- 
cute the  same  vibrations."  Bourseul  did  not  work  out  his 
ideas  to  a  practical  end. 

The  next  step  in  the  history  of  the  telephone  is  told  by 


ELECTRICITY    AND    MAGNETISM  277 

D.  E.  Hughes,  as  follows :  "  I  was  invited  by  his  Majesty  the 
Emperor  Alexander  II.  (of  Eussia)  to  give  a  lecture  before  his 
Majesty,  the  Empress,  and  court  at  Czarskoi  Zelo,  which  I  did; 
but  as  I  wished  to  present  to  his  Majesty,  not  only  my  own  tele- 
graph instrument,  but  all  the  latest  novelties,  Professor  Philipp 
Reis,  of  Eriedericksdorf,  Erankfort-upon-Main,  sent  to  Russia 
his  new  telephone,  with  which  I  was  enabled  to  transmit  and 
receive  perfectly  all  musical  sounds,  and  also  a  few  spoken 
words  —  though  these  were  rather  uncertain,  for  at  moments  a 
word  could  be  clearly  heard,  and  then,  for  some  unexplained 
cause,  no  words  were  possible.  This  wonderful  instrument 
was  based  upon  the  true  theory  of  telephony.  ...  Its  un- 
fortunate inventor  died  in  1874,  almost  unknown,  poor  and 
neglected;  but  the  German  government  has  since  tried  to 
make  reparation  by  acknowledging  his  claims  as  the  first  in- 
ventor, and  erecting  a  monument  to  his  memory  in  the  ceme- 
tery of  Eriedericksdorf."  ^  Eeis's  experiments  were  made  in 
1861. 

Eor  fifteen  years  electric  telephony  was  neglected,  then,  in 
1876,  Alexander  Graham  Bell  (born  1847)  invented  his  wonder- 
ful telephone,  which  is  still  used  at  the  present  time  as  the 
"receiver."  It  was  first  exhibited  publicly,  but  in  an  imper- 
fect form,  at  the  Centennial  Exhibition,  at  Philadelphia,  in  1876. 
Bell  was  born  at  Edinburgh,  in  Scotland,  and  took  up  his 
residence  in  the  United  States  in  1872.  In  a  lecture  delivered 
at  Cambridge  in  1878,  Clerk  Maxwell  said  that  when  the  news 
of  Bell's  invention  reached  England,  he  expected  the  new  in- 
strument to  surpass  the  siphon  recorder  in  delicacy  and  intri- 
cacy as  much  as  that  excels  a  common  bell-pull.  But  when 
the  instrument  appeared,  "  consisting,  as  it  does,  of  parts,  every 


1  Electrician  (London),  Vol.  34,  p.  637.    See  also  S.  P.  Thompson, 
Philipp  Beis,  London,  1883. 


278  A    HISTORY    OF   PHYSICS 

one  of  which  is  familiar  to  us,  and  capable  of  being  put  to- 
gether by  an  amateur,  the  disappointment  arising  from  its 
humble  appearance  was  only  partially  relieved  on  finding  that 
it  was  really  able  to  talk."  ^ 

Strange  to  say,  on  the  veijy  same  day  (February  14, 1876)  on 
which  Bell  patented  his  telephone,  Elisha  Gray  applied  for  a 
patent  for  an  instrument  of  a  similar  kind.  Later  one  com- 
pany took  up  the  patents  of  both  inventors. 

While  Bell's  instrument  seemed  perfect  as  a  "receiver," 
it  was  defective  as  a  "transmitter."  The  first  step  toward 
remedying  this  defect  was  the  invention  of  the  carbon  trans- 
mitter by  Edison  and  of  the  microphone  by  David  Edwin 
Hughes.  Edison's  invention  was  brought  out  in  1877  and  con- 
sisted of  a  vibrating  plate  abutting  against  a  carbon  button. 
The  transmitters  used  in  more  recent  telephony,  such  as 
Blake's,  Berliner's,  Hunnings's,  and  others  are  all  con- 
structed on  the  principle  of  loose  contact  involved  in  Edison's 
instrument.^ 

Hughes's  microphone  is  the  same  in  principle  as  Edison's 
transmitter,  but  its  arrangement  and  action  are  quite  different. 
In  1865  Hughes  had  experimented  on  Eeis's  telephone.  On 
hearing  of  Bell's  success,  he  resumed  his  investigation  and 
produced  the  microphone.  It  was  first  exhibited  in  1878  at 
his  rooms  to  a  company  including  Huxley,  Lockyer,  and 
W.  H.  Preece.  The  new  apparatus  was  of  the  most  primitive 
character,  "  consisting  of  a  child's  half -penny  wooden  money- 
box for  a  resonator,  on  which  was  fixed  by  means  of  sealing- 
wax  a  short  glass  tube,  filled  with  a  mixture  of  tin  and  zinc^ 
the  ends  being  stopped  by  two  pieces  of  charcoal  to  which 

1  Nature,  Vol.  18,  p.  160. 

2  Consult  W.  H.  Preece,  The  Telephone.  The  reader  will  find  much 
information  in  Thomas  Gray's  article,  "The  Inventors  of  Telegraph  and 
Telephone,"  JSmithsonian  Beport,  1892,  pp.  639-657. 


SOUND  279 

were  attached  wires,  having  a  battery  of  three  small  Daniell 
cells  —  consisting  of  three  small  jam-pots  —  in  circuit.  The 
wires  were  led  away  to  a  Bell  telephone  placed  in  an  adjoin- 
ing apartment.  The  money-box,  which  had  one  end  knocked 
out,  served  as  a  mouthpiece  or  transmitter,  while  a  Bell  tele- 
phone was  used  as  a  receiver.  Sounds  scarcely  audible  .  .  . 
to  the  unassisted  ear  were  ,  .  .  delivered  with  startling  loud- 
ness through  the  Bell  telephone."  ^ 

SOUND 

In  the  eighteenth  century  sound  was  studied  mainly  by  the 
musicians  and  mathematicians ;  in  the  nineteenth  century  it 
became  a  regular  branch  of  research  for  the  physicist.  The 
"  father  of  acoustics "  is  Ernst  Florens  Friedrich  Chladni 
(1756-1827),  born  in  Wittenberg.  His  father  educated  him 
for  law,  but  after  the  death  of  his  father  he  devoted  himself 
to  science.  His  reading  of  several  papers  on  sound  brought 
him  to  the  conviction  "that  in  that  more  remains  to  be  dis- 
covered, because  the  mathematico-physical  assumptions  are  far 
more  meagre  than  is  usual  in  science."  Euler's  and  Ber- 
noulli's mathematical  papers  led  him  to  investigate  sounding 
plates.  The  necessity  of  earning  a  livelihood  induced  him  to 
travel  in  order  to  give  art  performances  and  scientific  lectures. 
He  invented  a  new  musical  instrument,  the  euphonium,  on 
which  he  performed  during  his  travels  in  Germany,  France,  and 
Italy.  He  also  made  a  collection  of  meteorites.  "Inventive 
power,  ready  wit,  and  good  nature  distinguished  him  above  all."  ^ 

Chladni   experimentally  studied  the  vibrations  of   strings, 

1  Nature,  Vol.  55,  1897,  p.  497.  For  the  origin  and  development  of 
the  telephone  switchboard,  "the  brain  of  the  telephone  system,"  see 
Electrician  (London),  Vol.  34,  1895,  p.  895. 

2  ROSENBERGER,  III.,  p.   125. 


280  A    HISTORY    OF   PHYSICS 

rods,  and  plates.  "Chladni's  figures"  are  celebrated;  they 
are  formed  by  the  sand  collecting  at  the  nodal  lines  of  vibrat- 
ing plates.  When  Chladni,  in  1809,  exhibited  his  figures 
before  the  French  Institute,  they  created  great  interest  among 
the  members,  including  Laplace.  Napoleon  had  the  experi- 
ments repeated  for  him  at  the  Tuileries,  and  gave  Chladni 
6000  francs  for  the  purpose  of  enabling  the  latter  to  translate 
his  Akustik  (first  published  in  1802)  into  French.  Chladni 
discovered  the  longitudinal  vibrations  in  a  string  or  rod,  as 
well  as  their  application  to  the  determination  of  sound  velocity 
in  solids;  he  first  investigated  torsional  vibrations  in  rods, 
and  determined  the  absolute  rate  of  vibration  of  bodies.  He 
determined  the  velocity  of  sound  in  other  gases  than  air  by 
filling  organ  pipes  with  the  gas  and  then  determining  the 
resulting  pitch.  An  elegant  method  of  comparing  velocities 
in  gases  or  in  solids  was  invented  in  1866  by  A.  Kundt. 
"Kundt's  method"  has  been  generally  introduced  into  ele- 
mentary instruction. 

A  far-reaching  discovery,  as  important  in  light  as  in  sound, 
was  the  principle  of  interference  of  ivaves,  which  we  owe  to 
Thomas  Young.  He  explained  it  in  a  paper  of  1800,  and  later 
again  in  his  Lectures  on  Natural  Philosophy.  Wave  motion 
was  made  the  subject  of  careful  study  on  the  part  of  Wilhelm 
Weber  and  his  brother,  Ernst  Heinrich  Weber  (1795-1878),  who 
published,  in  1825,  their  work,  entitled  Wellenlehre. 

It  was  long  believed  that  in  liquids,  sound  waves,  consisting 
of  condensations  and  rarefactions,  could  not  travel  at  all,  for 
the  reason  that  liquids  appeared  to  be  incompressible.  The 
compressibility  of  water  had  been  the  subject  of  experimenta- 
tion on  the  part  of  the  Accademia  del  Cimento  in  Florence, 
sometime  between  1657  and  1667.  Hollow  spheres  of  silver 
were  filled  with  water,  closed  tight,  and  then  disfigured  by 
hammering.     The  water  was  forced  through  the  pores  of  the 


SOUND  281 

metal.  Apparently  water  is  incompressible.  Boyle  believed 
that  water  was  elastic,  but  could  not  establish  Ms  view  by 
conclusive  experiment.  In  1762  John  Canton  demonstrated 
before  the  Royal  Society  that  Avater  is  compressible,  but  his 
test  received  little  attention.  More  accurate  figures  on  the 
degree  of  compressibility  were  obtained  by  Oersted  about  1822. 
Like  Canton,  he  experimented  by  subjecting  the  vessel  contain- 
ing the  water  to  the  same  pressure  outside  as  inside,  thereby 
preventing  a  change  in  its  capacity.  His  results  indicated  a 
diminution  of  the  .000047th  part  of  the  original  volume  when 
the  pressure  was  increased  by  one  atmosphere.  A  somewhat 
larger  value  —  .0000513  —  was  obtained  in  1827  by  Jean  Daniel 
Colladon,  professor  of  mechanics  in  G-eneva,  and  Jacob  Carl 
Franz  Sturm  (1803-1855)  of  Geneva,  who,  after  1830,  was  pro- 
fessor of  mathematics  in  Paris.  These  co-workers  determined 
also  the  velocity  of  sound  in  water.  The  experiments  were 
made  on  Lake  Geneva,  between  Thonon  and  Eolle,  a  distance 
of  13,487  metres.  At  one  station  a  bell  was  placed  under 
water  and  struck  with  a  hammer ;  at  the  other  station  a 
specially  prepared  ear  trumpet  was  dipped  into  the  water. 
The  velocity  was  found  to  be  1435  metres  per  second.  Felix 
Savart  (1791-1841),  a  teacher  in  Paris,  and  later  conservator 
of  the  physical  cabinet  at  the  College  de  France,  showed  in 
1826  that  sound  waves  are  propagated  in  water  in  the  same 
way  as  in  solids.  Cagniard-Latour  succeeded  in  imparting 
sound  vibrations  to  water  by  means  of  the  siren.  This  ability 
of  the  instrument  to  cause  audible  sounds  in  water  led 
Cagniard-Latour  to  name  it  a  "  siren."  He  greatly  improved 
the  siren  and  its  mechanism  for  counting  vibrations.  This 
apparatus,  together  with  other  devices,  was  used  by  Savart  in 
determining  the  limits  of  audibility.  He  could  hear  tones  of 
bodies  vibrating  at  the  rate  of  24,000  or  48,000  per  second. 
The  lower  limit  he  placed  at  14  or  16  per  second. 


282  A   HISTORY    OF   PHYSICS 

A  new  epoch  in  the  history  of  the  science  of  sound  was 
created  by  Helmholtz,  who  in  1863  published  the  first  edition 
of  his  LeJire  von  den  Tonenvpjindungen.  The  third  G-erman 
edition  of  1870  was  translated  into  English  by  Alexander  J. 
Ellis  in  1875.  New  German  and  English  editions  have 
appeared  since.  Helmholtz  attributes  musical  tones  to  peri- 
odic motions  in  the  air ;  he  distinguishes  musical  tones  by  their 
Intensity,  Pitch,  and  Quality.  The  Quality  of  a  sound  he 
found  to  be  determined  by  the  "upper  partial  tones,"  which 
are  called  by  Tyndall  "  overtones."  Nearly  all  musical  tones 
possess  these  overtones,  the  number  and  relative  intensity  of 
which  determine  the  Quality.  G.  S.  Ohm  was  the  first  to  point 
out  that  there  is  only  one  form  of  vibration  Avhich  will  give 
rise  to  no  harmonic  upper  partial  tones,  but  consists  only  of 
the  prime  tone,  viz.  the  form  of  vibration  peculiar  to  the  pen- 
dulum and  tuning-fork.  Helmholtz  made  experiments  show- 
ing the  direct  composition  of  vowel  qualities,  which  were 
"essentially  distinguished  from  the  tones  of  most  other 
musical  instruments  by  the  fact  that  the  loudness  of  their 
partial  tones  does  not  depend  solely  upon  their  numerical 
order,  but  preponderantly  upon  the  absolute  pitch  of  those 
partials."  "If  only  the  unevenly  numbered  partials  are 
present  (as  in  narrow  stopped  organ  pipes,  pianoforte  strings 
struck  in  their  middle  points,  and  clarinets),  the  quality  of 
tones  is  hollow,  and,  when  a  large  number  of  such  upper  par- 
tials are  present,  nasal.  When  the  prime  tone  predominates, 
the  quality  of  tone  is  rich;  but  when  the  prime  tone  is  not 
sufficiently  superior  in  strength  to  the  upper  partials,  the 
quality  of  tone  is  poor."  ^  Helmholtz  devised  spherical  reso- 
nators by  which  he  analyzed  the  human  voice  and  musical 
tones  in  general.     He  also,  by  synthesis  of  sounds  from  tuning- 

1  Helmholtz,  Sensations  of  Tone.,  trans,  by  Ellis,  London,  1885, 
pp.  118,  119. 


SOUND  283 

forks,  operated  by  electromagnetic  apparatus,  succeeded  in 
producing  artificial  vowels,  which  were  close  imitations  of  the 
vowels  of  the  human  voice.  In  the  same  way  he  simulated 
the  quality  of  tone  of  organ  pipes,  although  the  "  whizzing 
noise,  formed  by  breaking  the  stream  of  air  at  the  lip,  is 
wanting  in  these  imitations." 

The  study  of  "beats"  led  Helmholtz  to  a  new  theory  of 
harmony.  Pythagoras  had  made  the  discovery  that  the  sim- 
pler the  ratio  of  the  two  lengths  into  which  a  string  is 
divided,  the  more  perfect  is  the  harmony  of  the  sounds  pro- 
duced by  these  two  parts  of  the  string.  Later  it  was  shown 
by  investigators  that  the  strings  act  in  this  way  because  of 
the  relation  of  their  lengths  to  the  rate  of  their  vibrations. 
Why  simplicity  should  give  pleasure  remained  an  enigma, 
even  after  Euler  had  declared  that  the  human  soul  takes  a 
constitutional  delight  in  simple  calculations.  Helmholtz,  by 
means  of  a  costly  polyphonic  siren,  which  he  had  constructed, 
experimented  on  beats.  In  the  case  of  two  simple  tones, 
the  number  of  beats  in  unit  time  is  equal  to  the  difference 
in  the  rates  of  vibration.  If  the  number  of  beats  is  33  per 
second,  then  the  dissonance  is  intolerable;  if  the  number  is 
smaller  or  larger,  the  effect  is  less  disagreeable ;  if  it  exceeds 
132,  then  the  unpleasantness  totally  disappears.  If  each  sound 
has  its  overtones,  then  the  question  of  harmony  or  dissonance 
is  more  complicated.  Beats  arising  between  fundamentals 
and  overtones,  or  between  the  overtones  themselves,  must  be 
brought  into  consideration.  It  is  found  in  a  general  way  that 
as  the  difference  in  pitch  of  two  musical  tones  is  so  varied 
that  the  disturbing  action  of  beats  becomes  more  and  more 
pronounced,  the  number  expressing  the  ratio  of  the  vibrations 
of  the  two  fundamentals  becomes  larger  and  larger.  Thus, 
Helmholtz's  theory  explains  how  it  is  that  the  simpler  ratios 
in  music  are  the  more  agreeable. 


284  A   HISTORY   OF   PHYSICS 

Helmholtz's  theory  of  harmony  met  with  much  criticism  oi, 
the  part  of  musicians  and  philosophers,  but  the  attacks  were 
unsuccessful,  and  the  opposition  to  it  has  disappeared. 

When  two  simple  musical  tones  are  sounded  together,  there 
occur  two  sound  phenomena :  (1)  the  heats  discussed  above, 
(2)  combinational  sounds.  The  latter  are  of  two  kinds :  the 
summational  tones,  discovered  by  Helmholtz,  and  the  differen- 
tial tones,  discovered  in  1744  by  the  German  organist,  Andreas 
Sorge,  and  again  by  the  celebrated  Italian  violinist,  Giuseppe 
Tartini.  Suppose  the  two  simple  tones  have,  respectively,  m 
and  n  vibrations  per  second,  then  the  rate  of  vibration  of  the 
differential  tone  is  m  —  n,  and  of  the  summational  tone  m  -\- n. 
To  produce  the  differential  tones  it  is  necessary  that  the 
primary  tones  be  of  considerable  intensity.  Helmholtz  used 
for  this  purpose  the  siren.  The  summation  tones  are  much 
more  difficult  to  observe.  They  were  predicted  and  discovered 
by  Helmholtz.  Rudolf  Konig,  the  celebrated  acoustic  in- 
strument maker  in  Paris,  entertained  views  which  in  some 
respects  were  contrary  to  those  of  the  great  German  investi- 
gator. Konig  held  the  opinion  that  when  rapid  beats  set  in 
they  themselves  give  rise  to  new  tones.  This  theory  was  not 
new ;  it  had  been  held  by  Lagrange  and  Thomas  Young,  but 
was  rejected  by  Helmholtz.  With  aid  of  large  tuning-forks  of 
his  own  make,  Konig  endeavoured  to  demonstrate  the  correct- 
ness of  his  view.  Konig  was  not  sure  that  he  could  detect 
with  his  tuning-forks  the  presence  of  summational  and  differ- 
ential tones,  but  he  claimed  to  hear  tones  of  the  rate  of  vibra- 
tion indicated  hj  m  —  vn  and  {v  +  1)  n  —  m,  where  m  >  n  and 
■y  is  a  whole  number,  so  that  vn  and  (y  -\-l)n  are  rates  of 
vibration  of  those  harmonic  overtones  of  the  lower  tones 
which  immediately  enclose  the  higher  tone.  W.  Voigt^  in 
1890,  concludes  that  both  the  combinational  tones  of  Helm- 
1  Wiedemann's  Annalen,  N.  F.,  Vol.  40,  pp.  662-660. 


SOUND  285 

holtz  and  the  beat  tones  of  Konig  can  theoretically  be  prO' 
duced,  and  that  the  one  system  or  the  other  will  predominate 
according  to  circumstances.  If  the  energy  of  the  two  vibrations 
approaches  equality,  combinational  tones  are  more  prominent ; 
otherwise  the  beat  tones  will  be  more  easily  heard. 

Konig  improved  an  invention  by  E.  Leon  Scott  of  the  year 
1859,  and  brought  forth  the  well-known  manometric-flame 
apparatus  for  the  analysis  of  sound.  The  phonograph  of 
Edison,  first  described  in  1877,  has  been  found  serviceable 
for  the  same  purpose. 

To  study  the  composition  of  vibrations,  Jul.  Ant.  Lissajous 
(1822-1880),  professor  at  the  College  Saint  Louis,  in  Paris, 
devised,  in  1855,  a  very  elegant  method.  The  two  vibrating 
bodies  (tuning-forks,  for  instance)  were  supplied  with  small 
mirrors.  A  ray  of  light  was  reflected  from  one  mirror  to  the 
other,  and  then  to  a  screen.  Usually  the  bodies  were  so 
placed  that  their  planes  of  vibration  were  perpendicular  to 
each  other.  The  curves  thus  traced  by  the  spot  of  light  on 
the  screen  are  known  as  "Lissajous's  figures,"  but  they 
had  been  discovered  long  before  in  the  United  States  by 
Nathaniel  Bowditcli  of  Salem,  Massachusetts.  In  1815  Pro- 
fessor Dean,  of  Burlington,  Vermont,  published  a  memoir,  on 
"  the  motion  of  the  earth  as  seen  from  the  moon,"  and  devised 
the  compound  pendulum  for  illustration,  which  is  supposed  to 
have  been  introduced  into  science  twenty-nine  years  afterwards 
by  Blackburn.  This  paper  induced  Bowditch  to  examine  the 
theory  of  the  motions  of  a  pendulum  suspended  from  two 
points,  and  to  make  a  few  experiments  to  test  his  theory.  He 
drew  figures  which  are  the  same  as  the  curves  of  Lissajous.^ 

1  Consult  J.  LovERiNG,  "Anticipation  of  the  Lissajous  Curves,"  in 
Proc.  of  the  Am.  Acad.,  N.  S.,  Vol.  8,  pp.  292-298;  for  Dean's  and 
Bowditcli's  papers  see  Memoirs  of  Am.  Acad,  of  Arts  and  Sciences,  1st 
Series,  Vol.  III.,  1815,  pp.  241,  413. 


THE    EVOLUTION    OF   PHYSICAL 
LABORATORIES 

It  would  be  useless  to  searcli  Antiquity  or  tlie  Middle  Ages 
for  laboratories  devoted  to  physical  investigation.  Before  tlie 
time  of  Galileo  and  of  Gilbert  of  Colchester  the  necessity  of 
experimentation  was  usually  overlooked.  Hard  thinking  was 
frequently  regarded  as  the  sole  requisite  for  scientific  dis- 
covery. It  was  not  until  the  time  when  Gilbert  constructed  a 
sphere  out  of  loadstone  and  showed  that  our  earth  behaves 
magnetically  much  like  his  miniature  representation  of  it, 
that  the  experimental  method  secured  a  firm  foothold  among 
physical  philosophers;  it  was  not  until  the  young  Galileo 
ascended  the  leaning  tower  of  Pisa  and  dropped  iron  balls  of 
different  weights  to  show  that  a  light  ball  will  fall  with  the 
same  acceleration  as  a  heavy  ball,  that  the  Aristotelian  idea 
concerning  physical  research  was  abandoned.  The  simul- 
taneous clang  of  those  two  weights,  as  they  struck  the  ground, 
"  sounded  the  death-knell  of  the  old  system  of  philosophy  and 
heralded  the  birth  of  the  new." 

It  is  amusing  to  observe  that  in  those  days  many  people 
reputed  for  wisdom  looked  upon  experiments  as  dangerous  to 
intellectual  and  moral  life.  In  a  history  of  the  Royal  Society, 
written  in  1667,^  the  author  deems  it  necessary  in  all  serious- 

1  Tho.  Sprat,  The  History  of  the  Boyal  Society  of  London^  1667,  pp. 
323,  328.  Consult  also  Robert  Boyle,  The  Usefulness  of  Experimental 
Philosophy ;  by  Way  of  Exhortation  of  the  Study  of  it,  in  three  parts, 
Oxford,  1663,  1671.  He  argues  that  experimental  science  does  not  lead 
to  atheism. 

286 


THE   EVOLUTION    OF   PHYSICAL   LABORATORIES      287 

ness  to  defend  experimentation,  arguing  that  '^  experiments 
will  not  injure  education ,''  tliat  "experiments  [are]  not 
dangerous  to  the  universities.''  The  arguments  were  neces- 
sary indeed,  for  the  Oxford  pulpit  declared  that  Eobert  Boyle's 
researches  were  destroying  religion  and  his  experiments  were 
undermining  the  universities.^ 

The  advent  of  the  experimentalist  marks  the  origin  of 
laboratories.  We  do  not  mean  laboratories  of  the  modern 
type.  Previous  to  the  nineteenth  century  all  of  them,  with 
hardly  any  exception,  were  private  laboratories  owned  by 
individual  investigators  or  their  patrons. 

For  chemistry  and  astronomy,  laboratory  facilities  were 
established  much  earlier  than  for  physics.  To  the  present 
day  the  word  "  Laboratorium  "  carries  in  Germany  the  mean- 
ing "  chemical "  laboratory.^  The  Middle  Ages  had  its  labora- 
tories for  alchemy  and  astrology.  The  search  for  the  elixir  of 
life  and  the  key  to  the  transmutation  of  metals  stimulated 
activity.  These  were  studies  congenial  to  the  avarice  of  the 
human  heart.  In  the  gallery  of  the  Louvre  in  Paris  is  a 
painting  by  the  Flemish  artist  Teniers,  the  elder  (?).  It 
represents  a  chemical  laboratory  of  the  sixteenth  century.® 
The  artist  portrays  a  large  basement  room  with  forge-furnaces. 
The  floor  is  covered  with  alembics,  crucibles,  and  retorts.  A 
group  of  enthusiasts  are  seated  round  one  of  the  tables.  Allow- 
ing somewhat  for  the  imagination  of  the  artist,  this  painting 
probably  pictures  to  us  the  more  luxurious  quarters  enjoyed 
by  alchemists  who  commanded  the  purse  and  protection  of 
some  powerful  patron.      The  majority  of  alchemists  experi- 

1  A.  D.  White,  Vol.  L,  p.  405. 

2  See  articles  "  Laboratorium  "  in  Brockhaus's  or  Meyer's  Konversa- 
tions-Lexikon. 

3  See  a  reproduction  of  the  picture  in  Johnson'* s  Universal  Cyclopoedia, 
article,  ' '  Laboratories. ' ' 


288  A   HISTORY   OF   PHYSICS 

mented  in  secret  retreats  far  from  luxurious.  Even  after  the 
complete  victory  of  the  inductive  method,  experimental  re 
search  was  usually  carried  on  in  rooms  intended  for  domestic 
or  commercial  purposes.  As  late  as  the  beginning  of  the 
present  century,  the  laboratory  of  the  most  distinguished 
chemist  of  his  day,  Berzelius,  was  his  kitchen,  in  which 
chemistry  and  cooking  Avent  on  together.  When,  through  the 
influence  of  Gilbert,  Galileo,  and  their  successors,  physics 
began  to  be  an  experimental  science,  it  was  generally  pursued 
in  the  same  apartments  as  its  sister  science,  chemistry. 
Formerly  specialization  was  less  marked  than  at  present  and  it 
was  no  uncommon  thing  for  a  scholar  to  be  a  master  in  several 
branches  of  science. 

The  earliest  physical  experiments  were  made  in  private 
laboratories.  The  investigator  usually  turned  part  of  his 
house  or  room  into  a  scientific  workshop.  When  Eobert  Boyle 
at  Oxford  worked  on  the  elasticity  of  gases,  proving  the  law 
which  bears  his  name,  he  employed  a  tube  of  such  length,  that 
he  "could  not  conveniently  make  use  of  it  in  a  chamber," 
and  he  was  "  fain  to  use  it  on  a  pair  of  stairs."  Newton  per- 
formed his  classic  experiments  on  the  dispersion  of  white 
light  into  its  component  colours  at  his  lodgings  in  Cambridge. 
Benjamin  Franklin,  after  experimenting  with  the  kite,  put  up 
an  insulated  iron  rod  at  his  house  in  Philadelphia,  in  order 
that  he  might  lose  no  opportunity  to  make  tests  whenever  the 
air  was  heavily  charged  with  electricity. 

Previous  to  this  century,  scientific  laboratories  existed  sim- 
ply for  original  investigation;  they  seldom  played  a  part 
in  elementary  or  in  higher  educatio7i.  Doubtless,  the  error 
of  this  practice  was  felt  by  many  teachers  and  scientists,  and 
chief  among  such  men  was  the  Moravian  educational  reformer, 
Johann  Amos  Comeiiius  (1592-1671),  who  said:  "Men  must 
be  instructed  in  wisdom  so  far  as  possible,  not  from  books, 


THE   EVOLUTION   OF    PHYSICAL   LABORATORIES      289 

but  from  the  heavens,  the  earth,  the  oaks  and  the  beeches; 
that  is,  they  must  learn  and  investigate  the  things  themselves, 
and  not  merely  the  observations  and  testimonies  of  other 
persons  concerning  the  things."  "Who  is  there,"  he  cries, 
"who  teaches  physics  by  observation  and  experiment,  instead 
of  by  reading  an  Aristotelian  or  other  text-book  ?  "  ^ 

Near  the  close  of  the  eighteenth  century,  Joseph  Priestley, 
the  discoverer  of  oxygen,  expressed  himself  as  follows:  "I 
am  sorry  to  have  occasion  to  observe  that  natural  science  is 
very  little,  if  at  all,  the  object  of  education  in  this  country. 
...  I  would  observe  that,  if  we  wish  to  lay  a  good  foundation 
for  a  philosophical  taste,  and  philosophical  pursuits,  persons 
should  be  accustomed  to  the  sight  of  experiments  and  processes 
in  early  life.  They  should,  more  especially,  be  early  initiated 
in  the  theory  and  practice  of  investigation,  by  which  many  of 
the  old  discoveries  may  be  made  to  be  really  their  own;  on 
which  account  they  will  be  much  more  valued  by  them."  - 

In  this  passage  Priestley  advances  an  idea  which  is  finding 
its  practical  realization  at  the  present  time,  since  it  is  only 
in  recent  years  that  there  have  been  established  laboratories 
for  pupils  of  high-school  grade,  in  which  the  young  students 
themselves  practise  physical  manipulation. 

We  have  seen  that  experimental  research  was  in  vogue 
earlier  in  chemistry  than  in  physics.  Chemistry  again  takes 
the  lead  in  the  establishment  of  laboratories  connected  with 
educational  institutions  and  planned  for  the  use  of  students. 
Why  this  lagging  behind  on  the  part  of  physics  ?  There 
appear  to  be  two  reasons.  In  the  first  place,  chemistry  ap- 
pealed more  directly  to  the  needs  of  practical  life.  A  know- 
ledge of  chemistry  was  indispensable  for  metallurgy.     On  the 

1  W.  H.  Welch,  "The  Evolution  of  Modern  Scientific  Laboratories," 
Electrician  (London),  Vol.  37,  1896,  p.  172. 

2  J.  Priestley,  On  Air,  Birmingham,  1790,  Vol.  I.,  p.  xxix. 


290  A   HISTORY   OF   PHYSICS 

other  hand,  the  age  of  steam  had  not  yet  arrived ;  electricit;y 
and  magnetism  were  sciences  still  in  their  infancy.  The 
second  reason  for  the  priority  of  chemical  laboratories  is  that 
they  are  less  expensive.  Earthen  vessels,  bottles,  test-tubes, 
a  stock  of  ordinary  chemicals,  are  not  expensive,  yet  go  a 
long  way  towards  equipping  a  chemical  laboratory.  Physical 
apparatus,  on  the  other  hand,  is  very  costly.  Three  hundred 
years  ago  an  air-pump,  thermometer,  and  telescope  were  ex- 
pensive luxuries ;  they  are  expensive  now.  One  hundred  and 
thirty  years  ago  Priestley  wrote,  "  Natural  Philosophy  is  a 
science  which  more  especially  requires  the  aid  of  wealth."  ^ 

Great  educational  movements  usually  begin  on  top.  The 
laboratory  method  of  instruction  was  first  introduced  into  the 
universities  and  thence  descended  to  the  more  elementary 
schools.  Lord  Kelvin^  claims  that  the  first  chemical  labora- 
tory for  the  instruction  of  students  was  established  at  the 
University  of  Glasgow,  prior  to  the  year  1831,  but  the  first 
laboratory  of  the  type  existing  to-day  was  apparently  estab- 
lished by  Liebig,  who,  in  1824,  became  extraordinary  professor 
of  chemistry  at  the  University  of  Giessen.^  Certainly  the 
new  movement  in  the  teaching  of  chemistry  was  started  in 
Germany  with  much  greater  momentum  and  with  more  far- 
reaching  influence  than  in  Scotland.  Students  from  all  parts 
of  the  civilized  world  flocked  to  the  little  university  in  the 
small  town  of  Giessen."*  Chemical  laboratories  were  soon 
built  in  Tubingen,  Bonn,  Berlin,  and  other  places. 

1  Joseph  Priestley,  History  of  Electricity ,  4th  ed.,  London,  1776, 
p.  XV. 

2  "  Scientific  Laboratories,"  Nature,  Vol.  31,  1885,  pp.  409-413. 

8  T.  C.  Mendenhall,  "The  Evolution  and  Influence  of  Experimental 
Physics,"  in  the  Quarterly  Calendar  of  the  University  of  Chicago,  Vol. 
III.,  August,  1894,  p.  10. 

*  Ira  Remsen,  "On  Chemical  Laboratories,"  Nature,  Vol.  49,  1894, 
p.  631. 


THE  EVOLUTION   OF   PHYSICAL  LABORATORIES      29\ 

The  earliest  American  institutions  in  wMcli  students  were 
sent  regularly  to  the  chemical  laboratory  to  make  their  own 
experiments  were  the  E-ensselaer  Polytechnic  Institute  at 
Troy,  New  York,  and  the  Massachusetts  Institute  of  Tech- 
nology in  Boston.  At  the  former,  laboratory  work  was  re- 
quired of  students  prior  to  1831,^  probably  from  its  very 
foundation  in  1824.  The  movement  was  independent  of  that 
at  Giessen.  At  the  Massachusetts  Institute  of  Technology 
perhaps  more  systematic  courses  were  given.  The  laboratory 
method  was  in  vogue  there  from  the  time  of  the  school's  foun- 
dation at  the  close  of  the  Civil  War.^ 

The  transition  from  private  laboratories  to  those  belonging 
to  universities  was  a  gradual  one.  Usually  it  took  effect  in 
this  wise.  A  few  teachers  permitted  the  most  enthusiastic 
and  promising  of  their  students  to  enter  their  private  labora- 
tories. Thus,  HeinricJi  Oustav  Magnus  (1802-1870)  in  Berlin 
threw  open  a  few  rooms  in  his  residence  for  physical  experi- 
mentation. Like  Liebig,  Magnus,  while  himself  a  student, 
had  drawn  his  inspiration  for  experimental  research  from 
Berzelius  and  Gay-Lussac.  The  influence  which  Magnus 
exerted  in  Germany  was  very  great.  "He  loved  youth,  and 
knew  how  to  make  himself  beloved  while  imparting  a  taste 
for  that  science  to  which  he  had  consecrated  his  life."  ^  He 
began  his  work  at  the  University  of  Berlin  in  1834  as  extraor- 
dinary professor  of  physics  and  in  1845  was  advanced  to 
the  position  of  ordinary  professor.  Some  idea  of  the  work 
in  his  private  laboratory  may  be  obtained  from  his  students. 
Says  one  of  his  American  pupils ;  "  While  I  was  engaged  there 
three  other  students  were  present,  one  occupied  by  an  investi- 

1  Science,  Vol.  20,  1892,  p.  53 ;  N.  S.,  Vol.  8,  1898,  p.  205. 

2  Science,  Vol.  19,  1892,  p.  351. 

3  "  Life  and  Labours  of  Henry  Gustavus  Magnus,"  Smithsonian  Beportt 
1870,  pp.  223-230. 


292  A   HISTORY   OF   PHYSICS 

gation  of  acoustics,  another  in  polarized  light,  and  a  third  in 
the  measurement  of  crystals  of  recently  discovered  chemical 
compounds."  ^  Among  the  greatest  of  his  pupils  who  experi- 
mented under  him  were  G.  H.  Wiedemann,  Helmholtz,  and 
Tyndall.  As  the  number  of  students  increased,  the  private 
laboratory  became  more  and  more  inadequate ;  the  university 
began  to  give  financial  aid  and  the  private  establishment  grew 
into  a  regular  university  institution.  By  this  process  the 
private  laboratory  of  Magnus  evolved  into  the  physical  lab- 
oratory of  the  University  of  Berlin,  which  was  opened  in 
1863.  A  similar  mode  of  evolution  can  be  traced  in  case  of 
Liebig's  chemical  laboratory  at  Giessen  and  Purkinje's  physio- 
logical laboratory  at  Breslau.^ 

Physical  laboratories  for  students  were  gradually  estab- 
lished in  connection  with  other  German  universities.  Thus, 
at  Heidelberg  one  was  opened  by  Philipp  Giistav  Jolly  (1810- 
1884)  in  1846.  It  consisted  of  two  rooms  in  what  was  origi- 
nally a  private  dwelling.^  In  1850  the  apparatus  was  moved 
to  somewhat  more  commodious  quarters,  in  which  later  Kirch- 
hoff  and  Bunsen  instituted  their  wonderful  researches  in  spec- 
trum analysis.  Keferring  to  these  new  quarters,  Quincke 
says:  "However  modest  this  institute  may  appear  to  the 
present  generation,  it  contained  the  only  physical  laboratory 
in  which  a  German  student  at  that  time  could  do  practical 
work."  If  Quincke  means  to  exclude  private  laboratories, 
then  this  statement  may  be  true,  but  students  had  been  drawn 
to  Berlin  to  work  in  the  private  laboratory  of  Magnus  long 
before  this.     Helmholtz  was  there  in  1847. 

1  A.  R.  Leeds,  "A  Laboratory  of  Experimental  Research,"  Jour. 
Franklin  Inst.  (3),  Vol.  59,  1870,  p.  210. 

2  Science,  Vol.  3,  1884,  p.  173. 

3  G.  Quincke,  Geschichte  d.  physik.^  Instituts  d.  Univ.  Heidelberg, 
Heidelberg,  1885. 


THE   EVOLUTION    OF   PHYSICAL   LABORATORIES      293 

The  University  of  Glasgow  in  Scotland,  which  was  men- 
tioned as  laying  claim  to  the  earliest  student's  laboratory 
in  chemistry,  is  also  a  candidate  for  the  honour  of  having 
first  given  laboratory  instruction  in  physics.  In  1845  Lord 
Kelvin  (Sir  William  Thomson)  became  professor  of  natural 
philosophy  at  Glasgow.  He  invited  some  of  his  students  to 
aid  him  in  his  original  researches ;  others  volunteered  for 
service.^  "  The  physical  laboratory  for  many  years  was  a 
disused  wine-cellar  in  the  old  university  buildings."  ^  Thus  old 
Bacchus  was  superseded  by  the  modern  goddess  Scientia. 
Experimental  research  was  carried  on  for  nearly  a  quarter 
of  a  century  in  this  room  and  in  another  one,  added  later. 
Finally,  in  1870,  the  university  was  moved  into  new  and  pa- 
latial buildings.  The  students'  laboratory  work  under  Kelvin 
was  mostly  original  investigation.  "  Their  interest  was  excited, 
was  kept  alive  by  their  constant  intercourse  with  the  guiding 
spirit  of  the  place,  and  their  zeal  was  such  that  .  .  .  the  labora- 
tory corps,  as  it  used  to  be  called,  has  been  known  to  divide 
itself  into  two  squads  —  one  which  worked  during  the  day,  the 
other  during  the  night,  for  weeks  together,  so  that  the  work 
never  paused."^  Neither  at  Glasgow  nor  at  Berlin  were  the 
laboratory  courses  in  physics  regular  prescribed  branches,  con- 
stituting an  integral  part  of  the  curriculum ;  entrance  into  the 


1  Says  Kelvin  :  "Three-fourths  of  my  volunteer  experimentalists  used 
to  be  students  who  entered  the  theological  classes  immediately  after  the 
completion  of  the  philosophical  curriculum.  I  well  remember  the  sur- 
prise of  a  great  German  professor  when  he  heard  of  this  rule  and  usage : 
'  What !  do  the  theologians  learn  physics  ?  '  I  said,  '  Yes,  they  all  do  ; 
and  many  of  them  have  made  capital  experiments.'  "  — Nature,  Vol.  31, 
1885,  p.  411. 

2  Nature,  Vol.  55,  1897,  p.  487. 

^  Ibidem,  p.  487.  Consult  also  Kelvin's  evidence  given  before  the 
Royal  Commission  on  Scientific  Instruction,  Minutes  of  Evidence,  1870, 
p.  332. 


294  A   HISTORY   OF   PHYSICS 

laboratory  was  purely  optional.  The  earliest  institution  in 
which  laboratory  physics  was  pursued  according  to  a  system- 
atic plan  for  its  educational  value,  and  was  a  required  part 
of  the  work  necessary  for  a  degree,  is,  we  believe,  the  Massa- 
chusetts Institute  of  Technology  in  Boston.  The  institution 
competing  with  it  for  that  honour  is  King's  College  in  London. 
New  England  and  Old  England  took  the  new  departure  about 
the  same  time.  Says  W.  G.  Adams:  "Professors  of  physics 
at  different  universities  have  usually  selected  their  best 
students  to  assist  them  in  their  private  laboratories,  to  the 
mutual  advantage  of  professor  and  student,  but  I  believe  that 
Professor  Clifton  was  the  first  to  propose,  more  than  three 
years  ago,  that  a  course  of  training  in  a  physical  laboratory 
should  form  a  part  of  the  regular  work  of  every  student  of 
physics.  This  system  was  adopted  and  at  once  put  in  action 
at  King's  College,  on  a  very  considerable  scale  for  a  college 
with  no  endowment  whatever,  and  has  been  working  now  for 
nearly  three  years.  Two  large  rooms  adjoining  the  museum 
of  physical  apparatus  were  fitted  up  for  a  physical  laboratory, 
and  a  third  room  was  built  for  a  store  and  battery  room.'^  ^ 

Robert  Bellamy  Clifton's  name  is  indeed  closely  identified 
with  instruction  in  experimental  physics  in  England.  He  was 
the  first  occupant  of  the  chair  of  natural  philosophy  at  Owens 
College,  Manchester.  After  his  removal  to  Oxford,  he  planned 
the  first  laboratory  in  England  "which  was  specially  built 
and  designed  for  the  study  of  experimental  physics.  It  has 
served  as  a  type.  Clerk  Maxwell  visited  it  while  planning 
the  Cavendish  Laboratory  (at  Cambridge),  and  traces  of  Pro- 
fessor Clifton's  designs  can  be  detected  in  several  of  our 
university  colleges."^    Maxwell  took   charge  of  the  depart 

^Nature,  Vol.  3,  1871,  p.  323. 

2  A.  W.  RiJcKER,  Nature,  Vol.  50,  1894,  p.  344. 


THE   EVOLUTION    OF   PHYSICAL   LABOEATOBIES      293 

inent  of  physics  at  Cambridge  University  in  1871,  and  his 
laboratory  was  built  in  1874.^ 

Both  at  Cambridge  and  Oxford  laboratory  practice  was 
optional,  and  the  number  of  students  undertaking  experimen- 
tal work  was  small.^  But  out  of  this  small  number  rose  some 
of  England's  physicists  of  the  present  time. 

In  the  early  part  of  this  century  France  was  the  great  centre 
for  experimental  research.  And  yet,  as  Professor  Welch  says, 
"France  was  long  in  supplying  her  scientific  men  with  ade- 
quate laboratory  facilities."  "  Bernard,  that  prince  of  experi- 
menters, worked  in  a  damp,  small  cellar,  one  of  those  wretched 
Parisian  substitutes  for  a  laboratory  which  he  has  called  ^  the 
tombs  of  scientific  investigators.' "  Gay-Lussac's  laboratory 
was  on  the  ground  floor  and,  to  protect  himself  from  the 
dampness,  he  wore  wooden  shoes.  But  in  spite  of  this,  French 
scientists  investigated  and  taught  with  enthusiasm.  Says 
Liebig  in  his  autobiography :  ^  "  The  lectures  of  Gay-Lussac, 
Thenard,  Dulong,  etc.,  in  the  Sorbonne,  had  for  me  an  inde- 
scribable charm.  .  .  .  French  exposition  has,  through  the 
genius  of  the  language,  a  logical  clearness  in  the  treatment  of 
scientific  subjects  very  difficult  of  attainment  in  other  lan- 
guages, whereby  Thenard  and  Gay-Lussac  acquired  a  mastery 
in  experimental  demonstration.  The  lecture  consisted  of  a 
judiciously  arranged  succession  of  phenomena,  —  that  is  to 
say,  of  experiments,  whose  connection  was  completed  by  oral 
explanations.  The  experiments  were  a  real  delight  to  me,  for 
they  spoke  to  me  in  a  language  I  understood." 

1  E.  T.  Glazebrook,  James  Clerk  Maxwell  and  Modern  Physics,  New 
York,  1896,  p.  73. 

2  Glazebrook,  op.  cit. ,  p.  76 ;  Minutes  of  Evidence  taken  before  the 
Boyal  Commission  on  Scientific  Instruction  and  the  Advancement  of 
Science,  1870,  pp.  387,  388,  28. 

3  Smithsonian  Beport,  1891,  p.  263. 


296  A   HISTORY   OF   PHYSICS 

Gay-Lussac  invited  Liebig  to  work  in  Ms  "  private  labora 
tory."  As  elsewhere,  there  were  in  Paris  no  public  laborato- 
ries for  students.  Original  workers  were  dependent  upon 
their  own  financial  resources.  Says  Arago :  "  At  the  end  of 
the  eighteenth  century  and  the  beginning  of  the  nineteenth, 
no  one  was  a  real  physicist  unless  possessing  a  valuable  col- 
lection of  instruments  well  polished,  well  varnished,  and 
arranged  in  glass  cases."  When,  in  1806,  Gay-Lussac,  who 
owned  only  a  few  instruments  of  research,  was  a  candidate  for 
the  Academy  of  Sciences,  he  had  much  trouble  in  overcoming 
these  prejudices.^  We  know  that  Dulong  expended  nearly  all 
his  wealth  on  apparatus.  Eresnel  conducted  his  immortal 
experiments  privately,  and  defrayed  from  his  own  resources 
the  heavy  expense  for  apparatus.  Foucault  carried  on  most 
of  his  experiments  at  his  own  residence.  On  one  occasion 
savants  flocked  to  the  humble  abode  of  Ampere  in  the  E.ue 
Fosses  Saint  Victor  to  see  a  platinum  wire,  as  soon  as  it  was 
traversed  by  an  electric  current,  set  itself  across  the  meridian.^ 

For  many  years  French  scientists  complained  of  meagre 
laboratory  equipment  and  lack  of  room,  until,  at  last,  Duruy, 
the  minister  of  public  instruction  (1864-1869),  undertook  to 
meet  the  requirements.  At  the  beginning  of  the  century, 
Germany  took  lessons  from  France;  at  this  new  period  the 
process  was  reversed.  Says  Professor  Welch:  "No  more 
unbiassed  recognition  of  the  value  and  significance  of  the 
German  laboratory  system  can  be  found  than  in  the  reports 
of  Lorain,  in  1868,  and  of  Wurtz,  in  1870,  based  upon  personal 
study  of  the  construction  and  organization  of  German  labora- 
tories." 

Two  decrees  of  July  31,  1868,  affirm  the  necessity  of  sup- 


1  Arago,  "  Eulogy  on  Gay-Lussac,"  Smithsonian  Meport,  1876,  p.  152. 
'Heller,  II.,  p.  609. 


THE   EVOLUTION    OF   PHYSICAL   LABORATOBIES      297 

plementing  the  lectures  on  science  with  practical  exercises  or 
manipulations.  The  same  decrees  provide  that  besides  the 
laboratories  for  students  there  shall  be  established  special 
laboratories  for  original  research  for  the  use  of  professors 
and  other  savants.  The  result  was  the  establishment  of  a 
large  number  of  laboratories  for  physics  and  for  other  sci- 
ences.^ Eeferring  to  these  changes,  M.  Darboux  wrote,  in 
1892 :  "  You  know  what  profound  transformations  have  been 
accomplished  in  these  establishments  [the  faculty  of  sciences] 
within  20  years.  Everywhere  the  buildings  have  been  recon- 
structed and  enlarged ;  they  have  been  supplied  with  large 
laboratories  for  the  experimental  sciences.  In  some  places 
these  are  still  too  small,  —  the  remedy  is  easy.  ...  A 
barrack  on  a  site  not  far  distant  is  sufficient.  Certainly,  we 
professors  of  the  faculties  of  Paris  will  never  forget  the  ser- 
vices rendered  to  superior  instruction  by  the  barracks  and 
halls  of  Gerson."^ 

A  physical  laboratory  was  founded  in  the  old  Sorbonne  in 
1868.  J.  Jamin  was  director  of  it  until  his  death  in  1886. 
In  1894  it  was  transferred  to  the  new  Faculty  of  Sciences 
and  was  reconstructed.  At  the  present  time  it  is  celebrated 
through  the  researches  of  its  director,  G.  Lippmann.^ 

In  the  United  States  the  growth  of  laboratories  during  the 
last  25  years  has  been  surprising.  As  already  noted,  the 
Massachusetts  Institute  of  Technology  took  the  initiative  in 
physics.  The  idea  of  giving  regular  laboratory  courses  in 
this  subject  to  large  classes  was  strongly  advocated  by  William 

1  Circular  of  Information^  Bureau  of  Education,  Washington,  D.C., 
No.  4,  1881,  p.  119. 

^Report  of  the  Commissioner  of  Education  ^  Washington,  D.C.,  1892- 
1893,  Vol.  1,  p.  234. 

8  A.  Berget,  in  La  Nature,  Vol.  26,  1898,  p.  225 ;  Nature,  Vol.  5a 
p.  12. 


298  A   HISTORY    OF   PHYSICS 

Barton  Rogers,  the  first  president  of  tlie  Institute.  In  draw- 
ing up  tlie  scope  and  plans  of  the  new  school,  in  1864,  he 
stated  some  of  the  leading  objects  of  such  a  laboratory.^ 

Ediuard  G.  Pickering  was  put  in  charge  of  the  department. 
In  Aprilj  1869,  J.  D.  Eunkle,  then  acting  president  of  the 
Institute,  wrote  as  follows:  "Pickering  has  drawn,  in  quite 
full  detail,  a  plan  for  the  physical  laboratory,  which  I  will 
send  you  before  long.  .  .  .  Pickering  is  very  anxious  to  be 
ready  by  October  next  to  instruct  the  third  year's  class  by 
laboratory  work;  and  if  an  experience  of  one  year  shall  be 
favourable,  as  I  feel  it  must  be,  we  can  then  gradually  enlarge 
our  facilities  and  take  in  the  lower  classes.  I  am  convinced 
that  in  time  we  shall  revolutionize  the  instruction  in  physics 
just  as  has  been  done  in  chemistry.''  ^ 

After  a  trial  of  a  little  over  one  year  Pickering  made  the 
following  statement :  "  The  great  difficulty  is  to  enable  20  or 
30  students  to  perform  the  same  experiment  without  duplicat- 
ing the  apparatus,  and  to  avoid  the  danger  of  injury  to  delicate 
apparatus.  Our  plan  is  this:  Two  large  rooms  (one  nearly 
100  feet  in  length)  are  fitted  up  with  tables,  supplied  with  gas 
and  water.  ...  On  each  is  placed  the  apparatus  prepared  for 
a  single  experiment,  which  always  remains  in  this  place,  thus 
avoiding  the  danger  of  breaking  it  in  moving.  A  full  written 
description  is  also  given  of  each  experiment."^  Several  other 
institutions,  Cornell  for  instance,  were  quick  to  follow  suit. 
In  the  article  just  quoted  Pickering  says:  "There  are  now 
(1871)  in  America  at  least  four  similar  laboratories  in  opera- 
tion or  preparation,  and  the  chances  are  that  in  a  few  years 
this  number  will  be  greatly  increased." 

1  Life  and  Letters  of  William  Barton  Bogers,  Boston  and  New  York, 
1896,  Vol.  II.,  p.  303. 

2  Op.  cit.,  Vol.  XL,  p.  287. 

s  Nature,  Vol.  3,  1871,  p.  241. 


THE  EVOLUTION   OF  PHYSICAL  LABORATORIES      299 

Notwithstanding  Pickering's  prediction,  tlie  vast  majority 
of  our  colleges  and  universities  failed  to  make  provision  for 
physical  laboratories  for  students  until  much  later.  In  this 
matter,  the  technical  schools  were  in  the  lead.  University  in- 
struction, as  distinguished  from  technical,  is  of  more  recent 
date.  In  1871  Harvard  College  had  no  instruments  for  elec- 
trical measurements,  and  Professor  Trowbridge  had  to  borrow 
from  Professor  Cooke's  private  collection  in  order  to  make 
some  tests  on  his  new  cosine  galvanometer.^  Most  of  the  large 
physical  laboratories  in  this  country  have  been  erected  and 
equipped  within  the  last  15  years,  but  now  "  we  have  some 
half-dozen  that  will  compare  with  any  university  laboratories 
in  Europe,"  2  with  the  exception,  perhaps,  of  a  few  like  the  one 
in  Zurich,  devoted  to  physics  and  electrotechnics,  which  has 
been  built  and  equipped  at  an  expense  of  3,000,000  francs. 

The  difficulty  of  arranging  laboratory  work  in  physics  for 
large  classes,  which  Pickering  endeavoured  to  overcome,  cannot 
be  said  to  have  been  removed  satisfactorily.  Some  of  our 
large  universities  devote  a  whole  building  to  the  purposes  of  a 
physical  laboratory  and  yet  teach  the  elementary  college 
physics,  required  of  all  the  students  for  a  degree,  by  text-book 
and  illustrated  lectures,  without  giving  the  pupils  an  oppor- 
tunity to  experiment  for  themselves.  The  teaching  force  and 
laboratory  facilities  are  inadequate  for  classes  of,  perhaps, 
several  hundred  members.  Experimental  work  is  done  only 
by  the  few  students  who  elect  more  advanced  physics,  or  by 
those  who  are  pursuing  technical  courses.  If  there  is  any 
truth  in  the  statement  that  even  Earaday  never  could  under- 

1  Science,  N.  S.,  Vol.  VIII.,  1898,  p.  204.  For  comparison  of  dimen- 
sions of  several  American,  English,  and  German  physical  laboratories,  see 
Nature,  Vol.  58,  1898,  pp.  621,  622. 

2  A.  G.  Webster,  «  A  National  Physical  Laboratory,"  The  Pedagogi- 
cal Seminary,  Vol.  II.,  1892,  p.  91. 


300  A   HISTORY   OF  PHYSICS 

stand  any  scientific  experiment  thoroughly  until  lie  had  not 
only  seen  it  performed  by  others,  but  had  performed  it  him- 
self, then  it  is  clear  that  the  above  method  is  far  from  ideal. 

There  are  two  distinct  methods  of  conducting  large  labora- 
tory classes  in  physics.  One  is  to  let  all  the  pupils  perform 
the  same  experiment  (measurement)  simultaneously,  each 
student  being  supplied  with  all  the  apparatus  necessary  for 
the  experiment.  The  second  method  is  to  let  each  student 
perform  a  different  experiment,  so  that,  at  one  time,  there  are 
as  many  different  experiments  in  progress  as  there  are 
students. 

The  first  method  has  the  great  advantage  of  permitting 
teachers  to  discuss,  once  for  all,  the  theory  of  the  experiment 
with  the  classes  as  a  whole,  instead  of  repeating  it  with  each 
student  individually.  Moreover,  it  is  easier  to  superintend  a 
large  class  when  all  are  working  at  the  same  thing  than  when 
each  is  performing  a  separate  task.  The  great  disadvantage 
of  this  mode  of  procedure  is  that  few  institutions,  if  any,  have 
the  resources  to  furnish  each  student  of  a  large  class  with  the 
same  instrument  of  precision.  Several  hundred  instruments 
of  the  same  kind  might  be  needed  for  each  experiment. 
Wherever  this  course  has  been  followed,  the  apparatus  has 
necessarily  been  of  cheap  quality  and  frequently  the  experi- 
mental work  has  lacked  the  desired  degree  of  accuracy. 

The  strong  point  of  the  second  method  is  that  it  necessitates 
no  duplication  or  multiplication  of  apparatus,  thus  making  it 
easier  to  equip  the  laboratory  with  instruments  of  high  qual- 
ity. Each  student  is  at  a  different  task.  The  members  of  the 
class  rotate  from  one  experiment  to  another  on  successive 
days.  There  is  less  opportunity  for  students  to  compare 
results,  each  pupil  being  thrown  more  upon  his  own  resources. 
It  is  an  individual  method,  calling  for  a  great  deal  of  "  elbow 
instruction."     A  teacher  cannot  at  one  time  take  care  of  as 


THE   EVOLUTION   OF   PHYSICAL   LABORATORIES      301 

many  pupils  by  this  method  as  by  the  first.  Again,  the  order 
in  which  the  experiments  are  taken  up  is  different  for  each 
student,  making  it  impossible,  as  a  rule,  to  take  the  experi- 
ments in  a  logical  succession. 

So  far  as  we  know,  there  are  few  colleges  and  universities 
in  which  either  of  the  two  methods  has  been  carried  out  in  its 
purity  with  all  students.  Usually,  in  large  classes,  a  combina- 
tion of  the  two  has  been  found  more  in  harmony  with  existing 
conditions ;  that  is,  the  class  is  divided  into  groups,  and  the 
experiments,  during  any  laboratory  period,  are  different  for 
each  group,  though  the  same  for  all  students  belonging  to  any 
one  group.  At  the  present  time  the  first,  or  class-method,  is 
the  prevailing  one  in  our  high  schools,  while  in  the  universities 
the  advanced  laboratory  courses  are  invariably  pursued  by  the 
second,  or  individual,  method. 

During  the  past  decade  laboratory  courses  have  been  devel- 
oped and  strengthened,  not  only  in  our  higher  institutions  of 
learning,  but  also  in  our  high  schools.  Many  high  schools 
to-day  are  better  equipped  than  were  some  of  the  prominent 
colleges  ten  years  ago.  The  experimentation  in  our  secondary 
schools  was  formerly  purely  qualitative,  but  now  the  quantita- 
tive work  is  being  emphasized  more  and  more. 

At  the  present  time  laboratory  instruction  in  physics  in 
secondary  schools  is,  perhaps,  more  fully  developed  in  the 
United  States  than  in  France  and  Germany.  M.  Darboux, 
dean  of  the  Paris  faculty  of  sciences,  reported  in  1892  as 
follows :  "  There  exists  indeed  in  every  lycee  a  physical 
cabinet,  but  the  instruments  to  be  put  into  the  hands  of  the 
students  for  the  manipulations  in  physics,  chemistry,  and 
natural  history  are  wanting."  ^     In  Germany  the  desirability 

1  Beport  of  the  Commissioner  of  Education,  Washington,  1892,  1893, 
Vol.  1,  p.  233. 


302  A  HISTORY  OF  PHYSICS 

of  letting  the  pupil  handle  apparatus  and  see  it  in  action  has 
been  abundantly  discussed.  Some  laboratories  have  been  con- 
ducted accordingly,  but  the  new  movement  is  less  general 
there  than  in  America.^ 

The  departure  in  the  direction  of  individual  laboratory  work, 
including  measurements,  for  secondary  schools  took  definite 
shape  in  the  United  States  when,  in  1886,  Harvard  College 
changed  its  entrance  requirements  in  physics.  "  It  was  now 
decided  to  establish  a  requirement  of  laboratory  work  to  be 
recommended  by  the  College  in  place  of  the  text-book  work, 
although  the  latter,  considerably  increased,  remained  as  an 
alternative  for  those  who  could  not  command  laboratory  facili- 
ties. It  was  soon  evident,  in  view  of  the  inexperience  of 
teachers  and  the  very  different  standards  and  methods  likely 
to  be  adopted  by  them,  that  a  special  course  of  experiments, 
carefully  thought  out,  .  .  .  was  needed  to  make  the  new  plan 
a  success."  A  pamphlet  was  issued  by  Harvard  in  1887, 
afterward  somewhat  revised,  under  the  title.  Descriptive  List 
of  Elementary  Physical  Experiments. 

In  recent  years  there  has  been  a  growing  demand  for  the 
establishment  of  national  laboratories  for  experimentation 
which  is  beyond  the  resources  of  laboratories  connected  with 
educational  institutions.  But  little  has  been  achieved  in  the 
way  of  securing  government  aid.  Yet  England,  Germany, 
and  France  have  institutions  which  in  part  fulfil  these  de- 
mands. England  has  the  Eoyal  Institution  with  the  new 
Davy-Earaday  Research  Laboratory;  Germany  has  its  Im- 
perial Physico-Technical  Institute  in  Charlottenburg ;  France 
for  100  years  has  had  its  Conservatoire  des  Arts  et  Metiers 

1  Consult  E.  J.  Goodwin,  "  Some  Characteristics  of  Prussian  Schools," 
Educational  Beview,  December,  1896;  also  a  critical  review  of  this  article 
in  Poske's  Zeitschrift  fur  den  Fhysikalischen  und  Chemischen  Unterricht, 
X.  Jahrgang,  1897,  pp.  161,  162. 


THE   EVOLUTION   OF   PHYSICAL   LABORATOBIES      303 

and,  for  some  years  past,  also  an  electrical  testing  laboratory 
in  Paris.^ 

Of  tlie  famous  laboratories  of  the  Eoyal  Institution  in 
London  an  English  writer  said  in  1870 :  ^^  Probably  a  greater 
part  than  to  the  universities  is  to  be  ascribed  in  the  spread 
and  development  of  modern  science  to  the  Eoyal  Institution, 
which  has  been  the  scene  of  the  teaching  and  labours  of 
the  three  by  far  greatest  philosophers  of  our  century,  of 
Young,  of  Davy,  and  of  Faraday."  ^  The  Briton  of  to-day 
speaks  of  it  as  the  "Pantheon  of  Science."  The  theatre, 
model-room,  and  workshops  of  the  Koyal  Institution  were 
erected  in  1800.  The  aim  of  the  Institution,  according  to  its 
founder.  Count  Eumford,  was  the  promotion  of  applied  science. 
It  originally  contained  a  workshop  for  blacksmiths  with  forge 
and  bellows.  All  sorts  of  models  of  machinery  were  brought 
together.  After  1802,  when  Eumford  left  England,  the  indus- 
trial element  declined,  and  original  research  in  pure  science 
predominated.  When  the  physical  laboratory  of  the  Eoyal 
Institution  was  erected,  there  was  nothing  equal  to  it  in  Eng- 
land. Nevertheless  it  was  very  unpretentious.  It  became 
memorable  for  the  brilliant  researches  of  Sir  Humphry  Davy, 
Earaday,  and  Tyndall.  For  70  years  it  remained  unaltered, 
and  at  the  end  of  that  time  it  was  very  inferior  to  the  new 
laboratories  in  Oxford,  Cambridge,  Manchester,  and  Glasgow.^ 
When  the  reconstruction  of  the  laboratories  at  the  Eoyal  Insti- 
tution came  under  consideration,  the  plan  was  at  first  opposed 
by  Tyndall.  He  almost  prayed  that  the  place  where  Davy 
and  Faraday  had  made  their  discoveries  might  be  preserved.'* 
But  improvements  were  necessary  and  were  made  about  1871. 

1  A.  G.  "Webster,  Pedagogical  Seminary,  Vol.  II.,  1892,  p.  101. 

2  C.  K.  Akin  in  Minutes  of  Evidence  .  .  .  on  Scientific  Instruction, 
1870,  p.  £0. 

3  Nature,  Vol.  7,  1872-1873,  p.  264.  *  Ibidem,  p.  264. 


304  A   HISTORY   OF    PHYSICS 

Through  the  generosity  of  Dr.  Ludwig  Mond,  the  labora- 
tories of  the  Eoyal  Institution  were  recently  enlarged,  and  a 
new  laboratory,  liberally  endowed  and  equipped  with  modern 
apparatus,  was  erected  immediately  adjoining  the  Eoyal  Insti- 
tution.  This  new  scientific  workshop,  called  the  "Davy- 
Faraday  Eesearch  Laboratory,"  was  opened  December  22, 
1896,  and  is  at  present  under  the  directorship  of  Lord 
Eayleigh  and  Professor  J.  Dewar.  It  is  "the  only  public 
laboratory  in  the  world  solely  devoted  to  research  in  pure 
science"  and  "open  to  men  and  women  of  all  schools  and 
of  all  views  on  scientific  questions."^ 

Tor  many  years  considerable  attention  has  been  given  to 
the  standardizing  of  apparatus  at  the  Kevj  Observatory, 
England.  Meteorological  instruments,  compasses,  photo- 
graphic lenses,  have  been  tested  and  verified.  Important 
researches  have  also  been  carried  on  there  on  terrestrial 
magnetism. 2  In  this  work  the  government  has  helped  but 
little.  It  furnished  the  site  and  the  use  of  an  old  building  j 
all  other  expenses  have  been  defrayed  through  private  bene- 
faction. 

Germany  has  become  the  envy  of  other  nations  because  of 
her  magnificent  new  Imperial  Physico-Technical  Institute  in 
Charlottenburg,  commonly  called  the  Reichsanstalt,  toward 
the  foundation  of  which  Werner  Siemens,  in  1884,  donated 
^  about  $125,000.  The  Eeichstag  voted  the  necessary  addi- 
tions to  this  sum.  New  buildings  were  provided,  and  in 
1888  Helmlioltz  was  made  director.  On  his  death,  in  1894, 
he  was  succeeded  by  F.  Kohlrausch.  The  Eeichsanstalt  has 
not  only  departments  equipped  for  purely  theoretical  research, 
but  also  others  devoted  to  the  study  of  problems  useful  to 
industry. 

1  N^ature,  Vol.  55,  1896,  p.  209.  2  :^ature,  Vol.  55,  1897,  p.  368. 


THE   EVOLUTION    OF    PHYSICAL   LABORATORIES      805 

Prance,  for  100  years,  lias  had  its  Conservatoire  des  Arts 
et  Metiers.  It  was  founded  in  the  old  priory  of  St.  Mar- 
tin des  Champs  in  1794,  as  a  public  repository  of  machines, 
models,  tools,  plans,  descriptions.  Erom  time  to  time  free 
courses  of  lectures  on  applied  science  were  given  to  working- 
men  and  artisans.  In  the  physical  line  a  beginning  was  made 
by  the  purchase  of  the  "Cabinet  de  Physique"  owned  by 
Charles,  and  by  the  establishment  of  the  chair  of  physics 
in  1829.  The  physical  equipment  has  been  enlarged  from 
time  to  time. 

Through  the  participation  of  18  nations,  an  International 
Committee  of  Weights  and  Measures  was  organized  in  1875. 
A  fine  laboratory  was  erected  in  the  Pavilion  de  Breteuil,  in 
the  Park  of  St.  Cloud,  near  Paris,  for  the  purpose  of  construct- 
ing international  standards  of  the  metric  system.^ 

1  A.  G.  Webster,  op.  cit.,  p.  94.  Por  information  on  the  proposed 
National  Physical  Laboratory  in  England,  consult  Electrician  (London), 
Vol.  41,  1898,  pp.  778-780. 


INDEX 


Aberration,  of  light,  79 ;  spherical,  84 ; 

chromatic,  84. 
Abney,  W.  de  W.,  180,  186. 
Absolute  temperature,  106. 
Absolute  thermodynamic  scale,  195, 

196,  207,  212. 
Absolute  units,  261-263. 
Absolute  zero,  106. 
Absorption,  of  radiation,  157, 174-179, 

181,  182 ;  electric,  247. 
Absorption  lines,  161-163.    See  Fraun- 

hofer  lines,  Spectrum  analysis. 
Academie  des  Sciences,  founded,  74. 
Accademia  del  Cimento,  91,  150,  280. 
Accelerated  motion,  33,  51,  53,  97, 100. 
Accumulators,  222,  223. 
Achromatic  lenses,  84,  85, 102-104, 154, 

155. 
Acoustics.     See  Sound. 
Action  at  a  distance,  61,  62,  131,  247, 

248,  251. 
Action  of  points,  121, 123. 
Adams,  J.  C,  58. 
Adams,  W.,  272. 
Adams,  W.  G.,  36,  294. 
^pinus,  128. 
Air,  resistance  of,  74-76;  weight  of, 

63-66,  68. 
Air-pump,  63,  67,  68,  70,  71,  74,  264. 
Air-thermometer,  90, 105, 109, 194-196, 

212. 
Airy,  275. 

Aitken,  J.,  205,  206. 
Akin,  C.  K.,  303. 
Albertus  Magnus,  22. 
Al  Biruni,  19. 
Albrecht,  G.,  118,  131. 
Alchemy,  287. 


Alembert,  D',  12,  99. 

Alexander  II.,  277. 

Al  Hasan.    See  Al  Hazen. 

Al  Hazen,  17*-19,  26. 

Al  Hazen's  problem,  18. 

Al  Khazini,  20. 

Alleyne,  12. 

Alternate  currents,  273. 

Amagat,  E.  H.,  200. 

Amalgamation  of  zincs,  220. 

Amber,  8,  117. 

Amontons,  65,  93,  105*,  106,  107,  109, 

196. 
Ampere,  A.  M.,  225*-227,  240,  248,  251, 

256,  273,  296. 
Ampere,  electric  unit,  262. 
Ampere's  rule,  226. 
Anamorphosis,  105. 
Anaximenes,  14. 
Anderson,  62. 
Andrews,  T.,  201*,  202. 
Anemometers,  47. 
Angelo,  Michel,  27. 
Angstrom,  A.,  159,  164,  165,  168,  169, 

170. 
Animal  electricity,  132-135. 
Anode,  first  use  of  word,  238. 
Anomalous  dispersion,  181-183. 
Anthemius  of  Tralles,  7. 
Antonius,  16. 
Antonius  de  Domini,  88. 
Aqueous  vapour,  absorbing  power  of, 

176,  177,  179. 
Arabs,  17-20,  26;  and  compass,  23. 
Arago,  76,  139,  142,  143,  146-150,  172, 

199,  225*,  226,  238,  240,  296. 
Aratus  of  Soli,  10. 
Arc  light,  157,  166,  270,  271. 


307 


308 


INDEX 


Archimedes,  3*-7,  16,  25. 

Argon, 1G5. 

Aristoplianes,  6. 

Aristotle,  l*-3,  5,  10,  12,  13, 19,  21,  25, 

26,  32,  33,  48,  56,  63,  101,  286. 
Arrhenius,  S.,  218,  220. 
Arsonval,  D',  212,  234. 
Ash,  135. 

Astatic  multiplier,  234. 
Astrolabium,  20. 
Astrology,  287. 
Astronomy,  17,  27-30,  99,  103. 
Athermanous,  183. 
Atkinson,  E.,  139,  187. 
Atlantic  cable,  275,  276. 
Atmosphere,  absorbing  radiation,  178, 

179. 
Atmospheric  electricity,  122-127,  132, 

133. 
Atomic  theory,  ancient,  12, 13;  modern, 

156,  198. 
Atoms,  comijound  nature  of,  165. 
Attraction,  magnetic,  8,  9,  23,  43,  45; 

electric,  43,   95,   118,  128,  131,  247; 

gravitational  {see  gravitation)  ;  of  a 

sphere,  59;   due  to  caloric,  115;   of 

light  corpuscles,  143. 
Atwood,  G.,  100*. 
Augustus,  emperor,  16. 
Ayrton,  W.  E.,  274. 

Baarman,  18. 

Babinet,  149,  184*. 

Bache,  241. 

Bacon,  Francis,  13,  48,  49,  71,  93. 

Bacon,  Roger,  23,  26*,  38. 

Badgeley,  205. 

Baillif ,  Le,  250. 

Baily,  W.,  272. 

Ball,  W.  W.  R.,  57,  59,  61,  75. 

Ballistic  curve,  75. 

Balloons,  196-198;  Biot  and  Gay-Lus- 

sac's  ascension,  197. 
Band  spectrum,  164,  165. 
Banks,  J.,  134. 
Barker's  water-mill,  5. 
Barnard,  F.  A.  P.,  224. 
Barometer,  63,  71,  74,  91,  96, 197. 
Barrow,  56,  82. 
Bartholinus,  E.,  81,  145. 
Barton's  buttons,  141. 


Bauer,  L.  A.,  94. 

Beats,  136,  283-285. 

Becquerel,   A.  C,  181,  219,  220,  233, 

250,  267. 
Becquerel,  E.,  158,  160, 183,  267. 
Becquerel,  H.,  267. 
Bede,  The  Venerable,  9. 
Bedell,  F.,  245. 
Belanger,  J.  B.,  52. 
Belfast  Address  (Tyndall's),  175,  252. 
Bell,  A.  G.,  176,  277*-279. 
Bell,  L.,  156,  169. 
Bellerophon,  horse  of,  9. 
Belli,  267. 
Bellin,  260. 
Benjamin,  Park,  9,  22,  24,  46,  94, 122, 

124,  125. 
Bennet,  A.,  130,  267. 
Bentlej",  R.,  56,  61. 
Benzenberg,  J.  F.,  75. 
Berget,  A.,  297. 
Bergman,  128. 
Berliner's  transmitter,  278. 
Bernard,  295. 
Bernoullis,  the,  99,  279. 
Bertelli,  45,  46. 

Berthold,  G.,  31,  36,  70, 114,  193. 
Berthollet,  156, 197. 
Bertrand,  149. 

Berzelius,  199,  217*,  288,  291. 
Bessard,  45. 
Bigelow,  F.  H.,  261. 
Biot,  139,  147*,  148,  181,  197. 
Biran,  Maine  de,  226. 
Birch,  61,  70,  75,  96. 
Black,  J.,  113,  115*  116. 
Blackburn,  H.,  62. 
Blackburn's  pendulum,  285. 
Blake's  transmitter,  278. 
Blanco,  A.,  45. 
Blathy,  O.  T.,  246. 
Bode,  P.,  18. 
Boerhave,  108. 
Boethius,  15. 

Boiling  points,  107,  110,  204. 
Bois-Reymond,  du,  62. 
Bolometer,  178,  179. 
Boltzmann,  130. 
Bonfort,  H.,  254. 
Bonstetten,  Baron,  184. 
Borelius,  39. 


INDEX 


309 


Borough,  46. 

Bose,  119. 

Bosscha,  274. 

BouUiau,  I.,  92,  93,  107. 

Bourseul,  C,  276. 

Bowditch,  N.,  285. 

Boyle,  R.,  25,  50,  67,  70*-74,  92,  93,  95, 

96,  99,  110,  141,  281,  286-288. 
Boyle's  Law,  72,  73,  199. 
Boys,  C.  v.,  234. 
Bradley,  James,  79*,  80, 151. 
Brahe,  Tycho,  29,  47. 
Branly,  E.,  256. 
Breusing,  A.,  24. 

Brewster,  Sir  D.,  85, 138, 148*,  157, 159. 
Bright,  C.  T.  and  E.  B.,  246,  275. 
Bright  line  spectra,  155,  157,  159,  161, 

163-165. 
British  Association,  organized,  148. 
British  Association  unit  of  resistance, 

262,  263. 
Brougham,  Lord,  129,  141,  148. 
Brugmans,  250. 
Bruno,  G.,  29. 
Brush,  C.  F.,  270. 
Bunsen,  R.  W.,  116,  154, 159, 160*,  161, 

168,  174,  292. 
Bunsen  burner,  160. 
Bunsen  cell,  221,  222. 
Burckhardt,  F.,  90. 
Biirgi,  Joost,  36. 
Burrows,  94. 
Buys-Ballot,  167*. 

Cables,  submarine,  275,  276. 

Caccini,  41,  78. 

Caesium,  161. 

Cagniard-Latour,  201*,  281. 

Cailletet,  L.,  200,  202,  203. 

Calibration  of  capillary  tubes,  110. 

Callendar,  H.  L.,  194. 

Galley,  J.,  112. 

Caloric,  doctrine  of,  102,  113-115,  137, 

189-193,  207. 
Calorimetry,  116. 
Camera  lucida,  153. 
Camera  obscura,  38. 
Campbell,  L.,  186,  188,  221,  251,  252. 
Canton,  J.,  118,  128,  281. 
Capacity  for  heat,  115. 
Capacity  of  condensers,  129. 


Carbon  disulphide,  176. 

Carbon  transmitter,  278. 

Cardan,  24*,  43,  45. 

Cardan's  suspension,  24. 

Carhart,  H.  S.,  222,  263. 

Carlisle,  Sir  A.,  135,  215. 

Carnot,  Sadi,  139,  206*-208,  211. 

Carthusian  monks,  120. 

Cassini,  G.  D.,  77,  80,  97. 

Castelli,  B.,64,  90,91. 

Catapult,  5. 

Cathode,  first  use  of  word,  238. 

Cavallo,  T.,  267. 

Cavendish,  H.,  128*-130,  196,  247. 

Celsius,  107,  110*  111. 

Centigrade  scale,  106,  110,  111. 

Centre  of  gravity,  3. 

Centre  of  oscillation,  54. 

Centrifugal  force,  35,  53,  59,  60;    ac= 

tion,  53,  54. 
Charles,  J.  A.  C,  196*,  197,  305. 
Charles,  Law  of,  106, 197. 
Charles  of  Anjou,  24. 
Chasles,  M.,  259. 
Chemistry,  17,  218. 
Chinese  and  the  compass,  22,  45,  82. 
Chladni,  E.  F.  F.,  279*,  280. 
Chladni's  figures,  280. 
Christian  V.,  78. 
Christiansen,  C,  182. 
Christie,  H.,  233. 
Christin,  111. 
Churchman,  J.,  260. 
Clair aut,  99. 

Clapeyron,  B.  P.  E.,  207. 
Clark,  L.,  222,  275. 
Clarke,  S.,  55. 
Clausius,  R.,  139,  208*,  214,  217,  218, 

254. 
Clement  IV.,  26. 
Cleomedes,  16*. 
Gierke,  A.  M.,  105,  167. 
Clifford,  W.  K.,  213. 
Clifton,  R.  B.,  164,  294*. 
Clocks,  5,  20,  34,  36,  54,  56. 
Clothes,  radiation  from,  177. 
Clouds,  formation  of,  205,  206. 
Glouet,  200. 
Cobalt,  magnetic,  249. 
CoefScient  of   expansion  of  air,  106, 

196. 


310 


INDEX 


Coherer,  256. 

Colding,  L.  A.,  209. 

Coleridge,  215. 

Colladon,  J.  D.,  281. 

Collinson,  P.,  120-123,  125. 

Colour-blindness,  186. 

Colour-box  (Maxwell's),  189. 

Colours,  15,  81-85,  102;  of  thin  plates, 
86, 141,  184 ;  of  polarization,  147  ;  in- 
fluence on  absorption,  177;  by  re- 
flection, 182;  by  transmission,  182; 
photography  in,  183,  184;  blindness 
to,  186 ;  mixing,  188,  189. 

Columbus,  45,  46. 

Combinational  sounds,  284,  285. 

Comenius,  J.  A.,  288*. 

Compass.    See  Mariner's  compass. 

Compressibility  of  water,  281. 

Concave  gratings,  170. 

Condensers,  electric,  129,  134. 

Condensing  electroscope,  134. 

Conductivity,  electric,  117,  182,  196; 
thermal,  182,  196. 

Conduction  currents,  253. 

Conservation  of  areas,  99. 

Conservation  of  centre  of  gravity,  99. 

Conservation  of  energy,  138,  207-214, 
219,  251. 

Conservation  of  mass,  138. 

Conservation  of  momentum,  99. 

Contact  theory,  134,  135,  219. 

Continuity  of  liquid  and  gaseous  state, 
201,202. 

Continuous  spectrum,  159, 166. 

Converters,  244-246. 

Cooke,  W.  F.,  274. 

Cooling,  rate  of,  93. 

Copernican  system,  27-31,  41,  46,  49, 
56. 

Copernicus,  N.,  28*,  29,  54. 

Coriolis,  52. 

Cornu,  A.,  151,  153. 

Corpuscular  theory,  47,  81,  86-88, 101, 
102,  115,  137,  143,  146-148,  156. 

Cortesius,  M.,  45. 

Cotes,  Roger,  61*,  62. 

Coulier,  205. 

Coulomb,  128,  129,  130*,  131,  135,  247 ; 
electric  unit,  262. 

"Couronne  de  tasses,"  135. 

Crest,  Micheli  du,  110. 


Cr^ve,  135. 

Critical  temperature  of  gases,  200, 202, 

204. 
Crookes,  W.,  162,  163, 181,  264*-266. 
Cros,  C,  184. 

"  Crown  of  cups,"  135,  220. 
Cryophorus,  153. 
Crystals,  light  phenomena  in,  81,  145, 

147,  148. 
Ctesibius,  5*. 
Cunaeus,  119. 
Curie,  S.,  267. 

Current,  strength  of  electric,  230. 
Current  electricity,  117,  132-135,  215, 

220,  256. 
Curved  pitching,  76,  82,  83. 
Cusa,  Nicolaus  de,  48. 
Cyclic  operations  in  engine,  207. 

Daft,  L.,  272. 

Daguerre,  158*. 

Daguerreotype,  158. 

D'Alembert,  12,  99. 

D'Alembert's  principle,  99. 

Dalence',  92,  110. 

Dalibard,  124, 125. 

Dalton,  J.,  13,  156,  198*,  206. 

Daniell,  J.  F.,  220*,  221,  243. 

Daniell  cell,  220,  221,  279. 

Daniell  hygrometer,  220. 

Danti,E.,47. 

Darboux,  297,  301. 

Dark    lines.      See   Absorption   lines, 

Spectrum  analysis. 
D'ArsouA^al,  212,  234. 
Da  Vinci,  27,  31,  48. 
Davy,  Sir  H.,  138,  171,  175,  192,  193, 

215*,  225,  235,  236,  270,  303. 
Dean,  285. 
Declination  (magnetic) ,  22,  45,  46,  94, 

260,  261. 
De  la  Rive,  A.  A.,  217*  219. 
De  la  Roche,  172,  173. 
Delaunay,  149. 
De  risle,  110. 
Dellingshausen,  N.  v.,  62. 
Delor,  125. 

Deluc,  J.  A.,  109*,  110,  116. 
Democritus,  7,  12*,  19, 114. 
De'ri,  M.,  246. 
De  Rochas,  5. 


INDEX 


311 


Desaguliers,  6,  111,  112. 

Descartes,  50-53,  63,  71,  75,  76,  82,  88, 

93 ;  vortices,  54-56. 
Despretz,  199. 
Dew,  formation  of,  10,  205. 
Dewar,  J.,  166,  188,  204,  265,  304. 
Diamagnetisin,   175,  250;    coining  of 

word,  250,  251. 
Diathermancy,  174, 183. 
Diatonic  scale,  12. 
Dielectric,  247,  248,  251-253. 
Differential  galvanometer,  233. 
Differential  thermometer,  171. 
Differential  tones,  284. 
Diffraction,  88,  141,  143,  158,  255. 
Diffraction  spectrum,  158. 
Digges,  L.,  38. 
Dimensional  equations,  263. 
Diogenes  Laertius,  11. 
Dip,  46,  261. 
Diplex  telegraphy,  274. 
Dirichlet,  L.,  227. 
Displacement  current,  253. 
Dispersion  of   light,  81-85,  103,  173, 

183 ;  anomalous,  181-183. 
Dissipation  of  energy,  208. 
Dissociation,  165,  218,  219. 
Dividing  engines,  170. 
Divisch,  P.,  126. 
Doberck,  W.,  79. 
Dobrowolsky,  273. 
Dodson,  260. 
Dolbear,  A.  E.,  246. 
Dollond,  John,  102,  103. 
Dollond,  Peter,  103. 
Doppler,  J.  C,  167* 
Doppler  effect,  166,  168. 
Double  refraction,  81,  145,  147. 
Double  stars,  168. 
Dove,  139,  231. 

Draper,  J.  W.,  49, 158*,  159,  168,  180. 
Drebbel,  C,  38,  90. 
Du  Crest,  Micheli,  110. 
Due,  260. 

Du  Fay,  117*,  118. 
Duhem,  P.,  49. 
Diihring,  E.,  51,  100,  214. 
Dulong,  P.  L.,  93,  194, 198*,  295,  296. 
Du  Moncel,  276. 
Duplex  telegraphy,  274, 
Duruy,  296. 


Dust,  188,  205,  206. 

Dynamic    electricity.      See    Current 

electricity. 
Dynamical  equivalent    of    heat,   192, 

195,  210-214. 
Dynamics,  30,  32,  99. 
Dynamo,  258,  269-271. 

Ebert,  H.,  254. 

Ebullition,  93. 

Eccentrics,  theory  of,  27. 

Echelon  spectroscope,  166. 

Edinburgh  Philosophical  Journal,  es- 
tablished, 148. 

Edison,  T.  A.,  270-272,  274,  278,  285. 

Eighteenth  century,  6,  56,  99-137,  279. 

Elasticity,  97,  98,  131,  253,  281. 

Electric  arc,  157,  166,  270,  271. 

Electric  discharges  through  partial 
vacua,  264-267. 

Electric  displacement,  253. 

Electric  influence  machines,  267,  268. 

Electric  lighting,  180,  246,  271. 

Electric  machines  (f rictional) ,  96,  118, 
128,  132. 

Electric  motor,  258,  272,  273. 

Electric  railway,  272. 

Electric  resistance,  178,  194,  230,  232, 
233;  of  metals  at  low  temperatures, 
204 ;  unit  of,  262,  263. 

Electric  sparks,  117,  122,  125,  126,  132, 
149,  181,  243,  245,  246,  254,  255. 

Electric  theories,  118,  121,  131,  137, 
251. 

Electricity,  among  Greeks,  8,  9;  Re- 
naissance, 41-47;  17th  century,  94- 
96;  18th  century,  117-135;  19th 
century,  137,  215-279. 

Electrics,  43,  117. 

Electro-chemistry,  135,  215-223, 

Electrolysis,  182, 215-223 ;  laws  of,  238. 

Electromagnetic  theory  of  light,  248, 
251,  253. 

Electromagnetic  waves,  253-256,  265. 

Electromagnetism,  137,  223  et  seq. 

Electromagnets,  240-243,  249,  269,  270, 
273. 

Electromotive  force,  in  cell,  219,  220, 
230 ;  standard  of,  222. 

Electron,  8. 

Electrophorus.  133.  267. 


312 


INDEX 


Electrostatics,  117,  129. 

Elizabeth,  Queen,  42. 

Ellis,  A.  J.,  11,  282. 

Ellis,  G.  E.,  189. 

Emission  theory,  47,  81,  86-88,  101, 
102,  115,  137,  143,  146-148, 156. 

Empedocles,  7*. 

Energy,  52,  53,  183,  214,  217 ;  dissipa- 
tion of,  208 ;  force  and  energy,  214. 
See  Eadiant  energy.  Radiant  heat. 

Eolipile,  5,  111. 

Epicurus,  114. 

Epicycles,  27. 

Eppur  si  muove,  31. 

Erman,  260. 

Ether  (luminiferous) ,  81, 137, 138,  151, 
185,  253. 

Ether-waves,  253-256,  265. 

Euclid,  7,  16. 

Eudoxus,  27. 

Euler,  L.,  62,  99,  102*,  103,  114,  279, 
283. 

Euphonium,  279. 

Evaporation  and  low  temperatures, 
203. 

Ewing,  J.  A.,257,  258. 

Exner,  F.,  181. 

Experimentation,  among  Greeks,  1, 
13,  14 ;  Middle  Ages,  21,  26 ;  Renais- 
sance, 27,  29,  32,  43,  48;  17th  cen- 
tury, 50, 51,  99 ;  18th  century,  99, 118, 
119,  124 ;  19th  century,  138,  139,  190, 
280.    See  Laboratories,  286-305. 

Extra  current,  238,  239,  242. 

Eye,  19,  104,  140,  187,  188;  energy 
needed  for  vision,  180. 

Fabbroni,  135,  219*. 

Fahrenheit,  G.  D.,  106*-109,  111. 

Falling  bodies,  2,  32-35,  51,  74,  97, 100. 

Farad,  electric  unit,  262. 

Faraday,  M.,  121,  130,  166,  175,  200, 
201,  217,  219,  221,  234*-240,  242-244, 
246-253,  255,  269,  273,  275,  303. 

Faure,  C.  A.,  223. 

Favaro,  A.,  33. 

Fay,  du,  117*,  118. 

Fechner,  219,  231. 

Ferdinand  II.,  91, 

Ferdinand  III.,  69. 

Fermat,  77. 


Ferranti,  S.  Z.  de,  246. 

Ferraris,  G.,  272. 

Ferrel,  W.,  75,  76. 

Fields,  S.  D.,  272. 

Fire-engine,  5. 

Fire-fly,  180. 

"Fits,"  theory  of,  88. 

Fitzgerald,  255. 

Fizeau,  H.  L.,  149, 150*,  151,  153, 168, 

180. 
Flames,  discharging  power  of,  117. 
Flaugergues,  193. 
Fleming,  J.  A.,  204,  244,  245,  255. 
Floating  bodies,  4. 
Florentine  Academy,  91,  92.    See  Ac- 

cademia  del  Cimento. 
Florentine  thermometers,  92. 
Fludd,  R.,  90. 
Fluorescence,  264. 
Focus  tubes,  267. 
Fog,  formation  of,  205,  206. 
Fontaine,  272. 
Fontana,  F.,  38,  39. 
Forbes,  G.,  151,  153. 
Forbes,  J.  D.,  188,  196*  252. 
Force,  52,  53,  63. 
Foucault,  J.  L.,  149*,  150-153,  158,163, 

180,  245,  296. 
Fourier,  139,  196. 
Fourth  state,  265,  266. 
Frankland,  E.,  175. 
Franklin,  B.,  120*-127,  177,  189,  240, 

288. 
Fraunhofer,  154*-157,  168. 
Fraunhofer  lines,  89,  154-171. 
Freezing  mixtures,  93. 
Freezing-points,  204. 
Freke,  122. 

Fresnel,  76,  1.39,  140,  142*-148,  296. 
Frogs'  legs,  132-135,  256. 

Gale,  274. 

Galileo,  21,  29,  31*-42,  49,  50,  51,  53, 
56,  63-65,  75,  77,  90,  91,  97,  99,  100, 
105,  286,  288. 

Galileo,  Vicenzo,  36. 

Galvani,  A.,  132*-135,  215. 

Galvanism,  132.  See  Current  electri- 
city. 

Galvanometer,  233,  2M,  276. 

Garnett,  W.,  186,  188,  221,  251,  252. 


INDEX 


313 


Gases,  laws  of,  71,  72,  106,  196-200; 
liquefaction  of,  200-204;  kinetic  the- 
ory of,  265. 

Gassendi,  71,  97*,  114. 

Gassiot,  J.  P.,  221,  264,  266. 

Gaulard  and  Gibbs,  246. 

Gauss,  C.  F.,  139,  259-261*  262,  273, 
276. 

Gay-Lussac,  J.  L.,  197*,  198,  295,  296. 

Gay-Lussac's  law,  106, 197,  291. 

Gebler,  K.  v.,  40. 

Geissler,  H.,  264*. 

Geissler  tubes,  256,  264. 

Gellibrand,  H.,  94*. 

Gerland,  E.,  6,  36,  74,  90,  91,  92,  106, 
109,  110. 

Gilbert,  Ph.,  149-151. 

Gilbert,  W.,  41*-47,  49,  50,  94,  96,  286, 
288. 

Gint,  W.,  274* 

Gioja,  Flavio,  24, 

Gladstone,  J.  H.,  159. 

Glaisher,  J.  W.  L.,  59. 

Glazebrook,  R.  T.,  58,  153,  251,  295. 

Gold-leaf  electroscope,  130, 134. 

Goldsmith,  140. 

Goodwin,  E.  J.,  302. 

Gordon,  A.,  118. 

Gordon,  130. 

Gothe,  268. 

Gould,  B.  A.,  169. 

Govi,  G.,  38. 

Graecus,  M.,  22. 

Graf,  J.  H.,  110. 

Gramme,  Z.  T.,  270,  272. 

Gratings,  156,  158,  168-170,  178, 181. 

Gravitation,  law  of,  54-62. 

Gravity,  25,  51,  61. 

Gray,  E.,  278. 

Gray,  J.,  268. 

Gray,  S.,  117*,  122. 

Gray,  T.,  278. 

Greeks,  1-14, 17,  21,  47. 

Green,  G.,  259*. 

Gregory,  James,  85. 

Grenville,  Sir  R.,  39. 

Griffiths,  E.  H.,  194,  212. 

Grimaldi,  82,  88*,  89,  143. 

Gronau,  J.  F.  W.,  75. 

Grothuss,  Ch.  J.  D.  v.,  215*-217. 

Grothuss's  chain,  216,  217. 


Grove,  W.  R.,  221*,  222. 

Grove  cell,  221. 

Guericke,  50,  66*-71,  74,  90,  96,  99. 

Guglielmini,  G.  B.,  75. 

Gunpowder,  22. 

Gunter,  E.,  94. 

Giinther,  S.,  36. 

Gustavus  Adolphus,  66 

Guyot  de  Provins,  23. 

Gyroscope,  150. 

Hadley,  J.,  104. 

Hale,  G.  E.,  105. 

Hall,  C.  M.,  104. 

Halley,  50,  57-59,  61,  79,  94,  95,  107; 
charts,  94,  260. 

Hamilton,  Sir  W.,  252. 

Hansteen,  C,  223,  260*. 

Harmony,  theory  of,  283,  284. 

Harmony  of  the  spheres,  12. 

Harriot,  T.,  39. 

Harris,  R.,  36. 

Harrison,  C.  W.,  246. 

Hartmann,  G.,  46. 

Haschek,  E.,  181. 

Hastings,  C.  S.,  105. 

Hauksbee,  95,  96,  118. 

Hauron,  D.  du,  184. 

Hawksbee.    See  Hauksbee. 

Heat,  89-93,  105-116,  189-214;  theory 
of,  93,  113,  137,  183,  207;  of  con- 
densation, 206;  dynamical  equiva- 
lent of,  192,  195,  210-214. 

Heath,  T.  L.,  3. 

Heaviside,  O.,  274. 

Hegel,  138,  139,  231,  268. 

Heiberg,  I.  L.,  7. 

Helium,  liquefied,  204. 

Heller,  A.,  14,  76,  97, 131,  296. 

Hellmann,  G.,  9,  10,  24,  44,  46-48,  65, 
90,  91,  94,  95. 

Helmholtz,  11,  12,  139,  161,  183,  186- 
188,  209,  211,  213*,  214,  218,  222,  251, 
254,  282-285,  292,  304;  Helmholtz's 
resonators,  282. 

Henry  IV.,  50. 

Henry,  J.,  231,  239*-244,  246,  269,  273, 
274. 

Henry,  Mary  A.,  240-243. 

Henry,  unit  of  self-induction.  263. 

Hering,  E.,  186,  187. 


314 


INDEX 


Heron  of  Alexandria,  5*,  6,  111,  112. 
Herschel,  J.  F.  W.,  42,  138,  157. 
Herschel,  Sir  W.,  101,  104,  129,  138, 

171*-173,  178. 
Hertz,  H.  E.,  248,  253,  254*-256,  266. 
Hertzian  waves,  181. 
Hieroglyphics,  140. 
Hieron,  King,  4,  5. 
High  vacua,  264-267. 
Hindus,  17,  22. 
Hipparchus,  27,  28. 
Hippo,  14. 
Hire,  La,  74. 

Hittorf,  W.,  164,  217*,  264,  265. 
Holcombe,  153. 
Holtz,  W.,  118,  267,  268. 
Holtz  machine,  254. 
Hooke,  R.,  47,  50,  57,  59,  60*,  71,  75, 

80,  82,  85,  86,  93,  136,  141. 
Hopkinson,  Th.,  121. 
Hoppe,  E.,  95,  96,  133,  135. 
Horror  vacui,  25,  63,  64,  66. 
Huggins,  W.,  166,  168*. 
Hughes,  D.  E.,  274,  277,  278. 
Hulls,  J.,  112. 
Hultsch,  4. 

Humboldt,  A.  v.,  135,  260,  261*. 
Humphreys,  W.  J.,  166. 
Hunning's  transmitter,  278. 
Huxley,  278. 
Huygens,  C,  36-38, 50-54, 57,  60, 62, 74, 

76,  80*,  81,  85,  86,  88,  92,  97,  99, 101, 

103,  107,  111,  144,  145. 
Hydraulic  organ,  5. 
Hydrogen,  liquid,  204. 
Hydrogen  thermometers,  195. 
Hydrometer,  6. 

Hydrostatic  paradox,  4,  25,  73. 
Hydrostatics,  4,  25,  62,  63,  73. 
Hygrometers,  48,  220. 
Hygroscope,  48. 
Hypatia,  6. 
Hysteresis,  magnetic,  258. 

Ice  calorimeter,  116. 

Iceland  spar,  81, 145. 

Illusion,  optical,  6, 15, 19. 

Imponderables,  137. 

Impulse,  52. 

Incandescence,  temperature  of,  159. 

Incandescent  lamp,  271,  272. 


Inclination  (magnetic),  46,  261. 
Inclined  plane,  30. 
"  Incompressibility  of  liquids,"  280. 
Indices  of  refraction,  155,  182, 
Induced  electric  currents,  237, 238, 244 ^ 

of  higher  order,  244. 
Induction,  electromagnetic,  236,  237, 

242,   252;    self-induction,   238,   239, 

242 ;  electrostatic,  247. 
Induction  coil.    See  Transformer. 
Induction,  method  of,  48,  49,  288. 
Inferential  comparator,  185. 
Influence  machines  (electric) ,  118, 267, 

268. 
Infra-red  rays,  171,  177-181, 185. 
Interference,  principle  of,  141-143, 146, 

280. 
International  ohm,  263. 
International  standards,  305. 
Inverse  squares,  law  of,  54-62,  131. 
Invisible  College,  71. 
Iron,  magnetic  property  of,  249,  257. 

258. 
Iron  filings,  16,  248,  256. 
Irvine,  116. 
Isenkrahe,  C,  62. 

Jablochkofie,  246. 

Jacobi,  C.  G.  J.,  214. 

Jacobi,  M.  H.,  262,  273. 

Jamin,  J.  C,  202,  246,  297. 

Jenkin,  W.,  238. 

Jevons,  49. 

Jewell,  L.  E.,  166. 

Joannides,  Z.,  38,  39. 

Johnson,  A.,  89. 

Jolly,  P.  J.,  210,  292*. 

Joly,  J.,  184. 

Jones,  B.,  223,  234,  236,  249,  250. 

Jones,  D.E.,  253,255. 

Joule,  J.  P.,  192,  193,  209,  210*-213. 

Joule,  unit  of  work,  263. 

Jowett,  B.,  7,  9. 

Jupiter,  satellites  of,  39,  40,  77,  78. 

Kahlbaum,  G.  W.  A.,  156, 157, 160. 

Kaleidoscope,  148. 

Kant,  12. 

Kastner,  51. 

Kathode,  first  use  of  word,  238. 

Kathode  rays,  266. 


INDEX 


815 


Kayser,  H.,  165. 

Keckerman,  25. 

Keeler,  J.  E.,  168. 

Kelland,  252. 

Kelvin,  Lord  (Sir  William  Thomson) , 

24,  62, 162, 163, 195,  207,  208,  210,  211, 

213,  214,  234,  259,  262,  267,  275,  276, 

290,  293. 
Kepler,  J.,  29*,  30,  38,  39,  41,  56. 
Kepler's  Laws,  30,  56,  57. 
Ketteler,  183. 
Kew  Observatory,  304. 
Kiel,  A.,  262. 
Kilogramme-metre,  52. 
Kinetic  theory  of  gases,  265. 
Kinnersley,  E.,  118,  120,  122,  124. 
Kirchhoff,  G.,  154,  157,  159, 160*,  161, 

162,  168,  233,  292. 
Kite  (Franklin's),  125. 
Klaproth,  22. 
Klein,  H.,  53. 

Kleist,  E.  G.  von,  118*,  119. 
Klingenstierna,  S.,  102. 
Kliigel,  G.  S.,  82. 
Knoblauch,  K.  H.,  174. 
Kohlrausch,  F.,  218*,  304. 
Konig,  R.,  284. 
Korteweg,  D.  J.,  76. 
Kramer,  P.,  76. 
Kronig,  139. 

Kundt,  A.,  182*,  183,  280. 
Kundt's  method,  280. 

Laboratories,  71, 118, 127, 129, 131, 139, 

175,  199,  203,  286-305 ;  in  education, 

288-305. 
Lagenbeck,  Max,  188. 
Lagrange,  36,  99,  139,  224,  258,  284. 
La  Hire,  74. 
Lambert,  106,  111. 
Lamont,  260. 
Lane-Fox,  271. 
Langley,  S.  P.,  101,  102,  113,  114,  172, 

173,  178*-180. 
Laplace,  97*,  99,  ^16,  139, 144, 147,  258, 

280. 
Larousse,  P.,  151, 172. 
Lasswitz,  12. 
Latent  heat,  113, 115. 
Lavoisier,  116*,  191. 
Laws  of  motion,  34,  35,  50,  51. 


Least  deviation,  155. 

Least  time,  principle  of,  77. 

Le  Baillif ,  250. 

Leclanche,  G.,  222*. 

Leeds,  A.  R.,  292. 

Legal  ohm,  263. 

Leibniz,  52,  53,  80,  86. 

Lemonnier,  L.  G.,  126. 

Lenard,  P.,  266. 

Lenses,  6,  38-40,  84,  102,  103, 155, 188, 
270 ;  achromatic,  84,  85, 102-104,  154, 
155. 

Lenz,  H.  F.  E.,  231,  269*. 

Leopold  de'  Medici,  91. 

Le  Sage,  C,  62. 

Leslie,  J.,  171*,  177. 

Lever,  among  Greeks,  2,  3. 

Leyden  jar,  119-121,  126-128,  132, 134, 
238,  247,  254 ;  discharge  oscillatory, 
244,  254. 

Libes,  A.,  73. 

Libri,  92. 

Lichtenberg,  G.  C,  267*. 

Liebig,  J.  v.,  49,  209,  290-293,  296. 

Light,  among  the  Greeks,  6, 7 ;  among 
Romans,  15,  16;  among  Arabs,  17; 
in  Middle  Ages,  26 ;  Renaissance,  37- 
41 ;  17th  century,  56,  57,  76-89,  96 ; 
18th  century,  101-105 ;  19th  century, 
137,  140-188,  248,  255;  diffraction, 
84,  141,  143,  158,  255;  dispersion, 
81-85, 103, 173, 183 ;  emission  theory, 
47,  81,  86-88,  101,  102,  115,  137,  143, 
146-148,  156;  "iits,"  88;  magneti- 
zation effect,  166  ;  refraction,  7,  16, 
19,  39,  76,  77,  81,  83,  85,  87,  103, 
171,  182,  255,  266,  267;  light  without 
heat,  180;  velocity,  Romer,  77,  78, 
Bradley,  79,  Fizeau,  Foucault,  and 
others,  148-153 ;  electromagnetic 
theory,  248,  251,  253;  rectilinear 
propagation  of  light,  81,  87,  144. 

Lightning,  95,  122-127. 

Lightning  rod,  123-127. 

Lines  of  force,  247,  248,  252,  269. 

Linne,  111. 

Linus,  F.,  71,  72,  86. 

Lippershey,  Hans,  37. 

Lippmann,  G.,  184,  297. 

Liquefaction  of  gases,  200-204,  236. 

Lissajous,  150,  285*. 


316 


INDEX 


Lissajous's  figures,  285. 

Liveing,  G.  D.,  166. 

Lloyd,  H.,  153. 

Loadstone,  8,  9, 16,  43-45,  286. 

Lockyer,  J.  N.,  165,  278. 

Lodestone.    See  Loadstone. 

Lodge,  O.,  41,  49,  56,  166,  255,  256. 

Lommel,  E.,  155,  227. 

Lorain,  296. 

Louis  XIV.,  77,  80. 

Louis  XVIIL,  143. 

Lovering,  J.,  152,  172,  285. 

Low  temperatures,  200,  201,  203,  204, 

249. 
Lucretius,  9,  15*,  16. 
Luminiferous  ether,  81,  137,  138,  151, 

185,  253. 

Mach,  E.,  31,  37,  49,  52-54,  64,  93,  100, 
136,  195,  209. 

Magdeburg  hemispheres,  68,  70. 

Magellan,  J.  H.  de,  100. 

Magnes,  the  shepherd,  8. 

Magnetic  circuit,  open  and  closed, 
246. 

Magnetic  Union  (German) ,  261. 

Magnetism,  among  the  Greeks,  8,  9; 
in  Middle  Ages,  23;  Renaissance, 
41-47;  17th  century,  94-96;  18th 
century,  117-135 ;  19th  century,  137, 
166,  204,  215-279;  theories  of,  256- 
259;  dip,  46,  261;  declination,  22, 
45,  46,  94,  260,  261 ;  hysteresis,  258. 

Magnetite,  8. 

Magnus,  H.  G.,  76,  139,  175,  176,  198, 
199,  217,  291*,  292. 

Mains,  E.  L.,  145*  146. 

Man,  A.  P.,  271. 

Manometric  flames,  285. 

Maps  of  solar  spectrum,  170. 

Marat,  115. 

Marci,  M.,  81*,  82. 

Marie,  M.,  52,  53,  73,  172. 

Mariner's  compass,  22-24,  45,  95. 

Mariotte,  50,  73*,  74. 

Marius,  S.,  38. 

Martine,  G.,  111. 

Marum,  200. 

Mascart,  E.,  168,  205,  263. 

Mass,  53. 

Masson,  62,  238,  264. 


Mathematics,  17,  27,  30,  99,  139,  196, 
248,  252,  259. 

Matter,  constitution  of,  12. 

Maxim,  H.  S.,  271. 

Maxwell,  J.  C,  52,  62,  121,  129,  130, 
186,  188,  221,  227,  248,  251*-255,  263, 
265,  277,  294. 

Mayer,  A.  M.,  186. 

Mayer,  R.,  209*,  210,  213,  214. 

Mayer,  T.,  131. 

Maze,  92,  93. 

McCormack.    See  Mach,  E. 

M'Cosh,  J.,  175. 

Mechanical  equivalent  of  heat,  192, 
195,  210-214. 

Mechanics,  among  Greeks,  1-6;  Ro- 
mans, 15;  Middle  Ages,  25;  Renais- 
sance, 30-37;  17th  century,  50-76; 
18th  century,  99, 100. 

Meibomius,  11. 

Melanchthon,  10. 

Melloni,  M.,  171,  172*-175, 177, 178. 

Mendenhall,  T.  0.,  290. 

Mercator,  G.,  44. 

Mercurial  phosphorus,  96. 

Mercury  thermometers,  93,  106,  107, 
109,  110,  177,  194, 195,  198,  212 ;  dis- 
placement of  zero-point,  193. 

Mersenne,  Marin,  65,  74,  85,  90, 
97. 

Metals  and  anomalous  dispersion  of 
light,  182. 

Meteorology,  Greek,  9,  10;  Renais- 
sance, 47,  48,  68;  17th  century,  91, 
92 ;  18th  century,  107,  176,  205. 

Metius,  A.,  38. 

Metre,  in  wave-lengths,  185. 

Mewes,  R.,  54,  62. 

Micheli  du  Crest,  110. 

Michell,  J.,  131. 

Michelson,  A.  A.,  151,  152*,  153,  166, 
184, 185. 

Micrometers,  168. 

Microphone,  278. 

Microscope,  37-39,  103, 168, 171. 

Miculescu,  212. 

Middle  Ages,  5,  15,  21-26,  287. 

Migration  of  ions,  216-220. 

Milhaud,  G.,  14. 

Mill,  J.  S.,  14. 

MiUer,  202. 


INDEX 


317 


Miller,  W.  A.,  158. 
Miller,  W.  H.,  162. 
Mirrors,   among  ancients,   7;   among 

Arabs,  18;   18th  century,  104,  105; 

19th  century,  143, 149,  151,  153,  285. 
Mitscherlich,  A.,  164. 
Mixing  colours,  188,  189. 
Mizauld,  48. 

Mohammed's  cofl&n,  9,  250. 
Mohler,  J.  F.,  166. 
Moll,  240. 
Moller,  62. 
Mollet,  206. 
Member,  A.,  109. 
Momentum,  35,  52,  53. 
Mond,  L.,  304. 
Monge,  200. 
Montgolfier,  196. 
Moon,  heat  from,  174;   temperature 

of,  179. 
Moons  of  Jupiter,  39,  40,  77,  78. 
Morland,  112. 
Morley,  E.  W.,151,  184. 
Morse,  S.  F.  B.,  273,  274* 
Motor,  electric,  258,  272,  273. 
Mottelay,  P.  F.,  42. 
Mount  Whitney  experiments,  179. 
Mountain,  260. 
Muir,  P.,  218. 
Multiple  spectra,  164. 
Mundane  virtues,  96. 
Murray,  270. 
Musschenbroek,  111,  119*,  120, 

Napoleon  I.,  135,  280. 

Napoleon  III.,  150. 

Natterer,  J.  A.,  200,  201. 

Neckam,  A.,  23. 

Neef,  245. 

Negative  electricity,  121. 

Nernst,  W.,  218-220. 

Neumann,  E.  F.,  139. 

Newcomb,  S.,  152*. 

Newcomen,  T.,  112,  113,  153. 

Newton,  Sir  I.,  47,  50,  51,  53-55,  56*- 
62,  74,  80,  81-89,  93,  95,  97,  99,  101, 
107,  115,  138,  139,  141,  145,  181,  188, 
224,  288. 

Nichols,  E.  F.,  181. 

Nicholson,  W.,  135,  215,  267*. 

Nickel,  magnetic  property  of,  249. 


Nicol,  W.,  249,  251. 
Nicol  prism,  249,  251. 
Nicomachus,  11,  12. 
Niepce,  J.  N.,  158* 
Nineteenth  century,  137-305. 
Nobert,  F.  A.,  168,  169. 
Nobili,  L.,  173*,  233,  234. 
Noble,  W.,  97,  136. 
Nollet,  J.  A.,  109,  117, 119, 122. 
Normal  spectrum,  178. 
Norman,  R.,  46. 
Northmore,  200. 
Norwood,  58. 

Oersted,  H.  C,  223*-226,  281. 
Oersted's    experiment,  223,  224,  233, 

236,  237,  268. 
Ohm,  G.  S.,  219,  227*-232,  241,  282. 
Ohm,  unit  of  resistance,  262. 
Ohm's  law,  130,  228-231,  259. 
Olzewski,  K.,  195,  204. 
One-fluid  theory,  121,  137. 
Optical  illusions,  6,  15,  19. 
Optical  mineralogy,  148. 
Optics.     See  Light. 
Organ  pipes,  136,  184,  280,  282. 
Osmotic  pressure,  218-220. 
Ostwald,  W.,  200,  216-218,  220,  224. 
Ostioald's  Klassiker,  3d,  36,  64,  67,  77, 

80,  107-109,  116,  131,  132,  194,  197- 

199,  206,  213,  224. 
Overtones,  97,  136,  282-285. 
Oxygen,  and  colour  of  sky,  188. 

Pacinotti,  A.,  270, 
Page,  CO.,  244*,  245. 
Palladium,  153. 
Palmer,  C.  S.,  220. 
Papin,  D.,  74,  112. 
Parabolic  mirrors,  7,  18,  26. 
Parallel  currents,  226. 
Parallelogram  of  forces,  1,  30,  35. 
Paramagnetic,  coining  of  word,  250; 

251. 
i  Pardies,  86. 
Parry,  C.  H.  H.,  12. 
Pascal,  50,  62*,  63,  65,  66. 
Pascal's  Law,  62. 
Patterson,  R.,  127. 
Paul,  Father,  90. 
Peacock,  G.,  87, 105,  141,  144. 


318 


INDEX 


Peirce,  C.  S.,  169,  184. 

Peltier,  J.  C.  A.,  269*. 

Peltier  effect,  269. 

Pemberton,  57. 

Pendulum,  35, 53,  54,  282 ;  and  rotation 

of  earth,  150 ;  compound,  285. 
Percussion,  74. 
Peregrinus,  23,  24,  44-46. 
Permanent  gases,  201,  202. 
Permeability,  258,  259. 
Perpetual-motion  machines,  23, 91, 207. 
Peter  de  Maricourt.     See  Peregrinus. 
Petit,  A.  T.,  93,  198*. 
Petit,  Pierre,  65,  74. 
Pfaff,  C.  H.,  219. 
Philolaus,  28. 
Phlogiston,  114,  137, 192. 
Phonograph,  285. 
Phosphorescence,  96,  266. 
Photography,  invention  of,  158;  with 

X-rays,  267 ;  in  natural  colours,  183, 

184. 
Physical   laboratories,    286-305.     See 

Laboratories. 
Physical  Society  of  Berlin,  139. 
Picard,  Jean,  58,  59,  78,  96,  97. 
Pickering,  E.  C,  168,  298. 
Pictet,  K.,  202-204. 
Pictet,  M.  A.,  192. 
Pierre,  I.,  194. 
Piggot,  Th.,  97,  136. 
Pitch  and  rate  of  vibration,  97. 
Planta,  M.,  118. 
Plante,  G.,  222*. 
Platinum  thermometers,  194. 
Plato,  7,  63. 
Platonists,  7. 
Playfair,  J.,  56. 
Pliny,  8,  12,  15*. 
Plucker,  J.,  159,  164,  264. 
Pllicker  tubes,  158,  164. 
Poggendorfe,  J.  C,  10,  76,  91,  139,  219, 

228,  231,  246. 
Poincare,  253,  255. 
Poinsot,  143. 

Poisson,  76,  131, 144,  248,  257* 
Polarity  of  magnets,  9. 
Polarization  of  cells,  220,  222. 
Polarization  of  dielectric,  252,  253. 
Polarization  of  light,  81,  143,  145,  147, 
148,  249,  251,  255,  266,  267. 


Polyphase  motors,  272. 

Poncelet,  J.  V.,52,53. 

Pope,  F.  L.,  272. 

Porphyry,  11. 

Porta,  B.,  38,  43,  46,  48. 

Portsmouth  Collection,  58. 

Positive  electricity,  121. 

Potassium,  discovery  of,  215. 

Potential,  electric,  130,  134,  219,  259, 
267. 

Potter,  H.,  112. 

Pouillet,  C.  S.  M.,  234*. 

Power  transmission,  electric,  272,  273. 

Preece,  W.  H.,  278. 

Pressure,  osmotic,  218. 

Preston,  Th.,  88,  153,  200,  255. 

Preston,  Tolver,  62,  208. 

Priestley,  82,  118,  119,  124,  126,  289, 
290. 

Primary  colours,  185,  186. 

Prismatic  spectrum,  178,  180. 

Projectiles,  34,  35,  50,  74,  75. 

Proust,  156. 

Ptolemaic  System,  27-30,  54,  56. 

Ptolemy,  Claudius,  7*  18,  28. 

Pulleys,  30. 

Pump,  5,  25,  63,  67,  70,  203.  See  Air- 
pump. 

Purkinje,  292. 

Pyrometer,  111,  194. 

Pythagoras,  11*,  12, 14,  283. 

Pythagoreans,  7,  28,  29. 

Quadruplex  telegraphy,  274. 
Quality  of  tone,  282,  283. 
Quartz  fibres,  234. 
Quincke,  G.,  160,  218,  292. 

Eadiant  energy,  137,  155-182,  253-256, 
265-267 ;  visual  effect  of,  180 ;  nomen- 
clature, 183. 

Kadiant  heat,  171-176, 179, 183,  255. 

Radiant  matter,  265. 

Radiation,  power  of,  159, 176. 

Radiography,  267. 

Radiometer,  181,  265. 

Railways,  electric,  272. 

Rain,  formation  of,  205,  206. 

Rainbow,  15,  26,  81,  88. 

Rain-gauge,  91. 

Raleigh,  Sir  W.,  39. 


INDEX 


319 


Ramsay,  W.,  202. 

Ramsden,  103,  118. 

Ramus,  P.,  21. 

Rankine,  W-  J.  M.,  208*  214. 

Raphael,  27. 

Rayleigh,  Lord,  188,  222,  263,  301. 

Reaumur,  107,  109*,  110,  111,  119. 

Rectilinear  propagation  of  light,  81, 

87,  144. 
Reflection,  laws  of  optical,  7,  17,  81, 

87,  145,  171,  182,  255,  266,  267. 
Refraction,  7,  16,  19,  39,  76,  77,  81,  83, 

85,  87,  103,  171,  182,  255,  266,  267; 

indices  of,  155,  182. 
Regnault,  H.  V.,  194, 198, 199*  208. 
Reich,  F.,  75. 
Reichsanstalt,  218,  304. 
Reis,  P.,  277*,  278. 
Remsen,  I.,  290. 
Renaissance,  27-49,  287. 
Repulsion,  electric,  9,  95,  118, 128, 131, 

247  ;  magnetic,  9,  16,  250. 
Resinous  electricity,  118. 
Resistance,  electric,  178,  194,  230,  232, 

233 ;  of  metals  at  low  temperatures, 

204 ;  unit  of,  262,  263. 
Reversible  engine,  207. 
Rey,  Jean,  90,  91. 
Rheostat,  232. 
Rhodium,  153. 
Ricci,  M.  A.,  65. 
Richer,  Jean,  53. 
Richmann,  G.  W.,  126. 
Riemann,  B.,  62. 
Riess,  139. 
Righi,  A.,  256. 
Ring  armature,  270. 
Ritchie,  E.  S.,  246*. 
Ritter,  J.  W.,  172*,  181,  215,  219,  222, 

268,  270. 
Robel,  E.,  136. 
Robert,  196. 
Robinson,  J.,  136. 
Rochas,  de,  5. 
Roche,  De  la,  172, 173. 
Rock-salt,  174. 
Rogers,  W.  A.,  171*. 
Rogers,  W.  B.,  298. 
Rohault,  J.,  55. 
Romans,  15,  16,  21. 
Rumer,  Olaf,  77*-80,  97,  107, 108,  151. 


Rontgen,  \V.  K.,266. 

Rontgen  rays,  266,  267. 

Rood,  O.  N.,  169,  186; 

Roscoe,  H.  E.,  160,  164,  198. 

"  Rose  of  the  Winds,"  24. 

Rosenberger,  15,  20,  22,  59,  75-77,  88, 

91,  102,  136,  155,  157,  160,  165,  172, 

199,  206,  262,  264,  279. 
Rosse,  Lord,  104. 
Rotary  field  motor,  273. 
Rotary  polarization,  147. 
Rotation    of    plane   of    polarization, 

249. 
Rousseau,  225. 
Roux,  le,  182. 
Rowland,  H.  A.,  170*,  195,  212, 224,  259, 

263. 
Royal  Institution,  140,  175,  176,  190, 

204,  210,  235,  236,  247,  303,  304. 
Royal  Society,  organized,  71. 
Rubens,  181. 
Rubidium,  161. 
Rucker,  A.  W.,  211,  261,  294. 
Rudberg,  F.,  198*. 
Ruhmkorff,  245*,  246. 
Ruhmkorff's  coil,  245,  246,  264,  267. 
Ruling  machines,  169, 170. 
Rmnford,  Count,   115,  140,  189*-193k 

303. 
Runge,  C,  165. 
Runkle,  J.  D.,  298. 
Ruoss,  H.,  105. 
Russell,  R.,  205. 
Rutherfurd,  L.  M.,  169*,  184. 
Rysaneck,  A.,  62. 

Sabine,  260,  261*. 
Sage,  C.  Le,  62. 
Saigey,  250. 
Samothracian  rings,  9. 
Sanctorius,  90. 
Sanson,  L.  J.,  187. 
Saturn  seen  "  threefold,"  41. 
Saturnian  satellites,  104. 
Sauveur,  J.,  136*. 
Savart,  F.,  281*. 
Savery, 112. 
Sawyer,  W.  E.,  271. 
Saxton,  J.,  158. 
Schaffers,  V.,  268. 
Schaik,  W.  C.  L.  v.,  36. 


320 


INDEX 


Schelling,  138,  268. 

Schmidt,  G.  C,  267. 

Schuaase,  L.,  18. 

Schott,  194. 

Schott,  Kaspar,  67. 

Schramm,  62. 

Schuck,  A.,  23. 

Schiilke,  A.,  248. 

Schweigger,  J.  S.  C,  233*. 

Schwenter,  90. 

Scott,  E.  L.,  285. 

Screw,  170. 

Screw  of  Archimedes,  5. 

Seasons,  variation  of,  29. 

Secondary  batteries,  222,  223. 

Seebeck,  T.  J.,  227,  248,  250,  268*. 

Selective  absorption,  178. 

Self-induction,  238,  239,  242. 

Seneca,  15*,  81. 

Servus,  H.,  38,  39,  85,  103. 

Seventeenth  century,  48,  50-99,  137. 

Shaw,  P.,  95. 

Short,  103,  104*. 

Shuttle  armature,  269. 

Siemens,  Werner,  232,  233,  262*,  269, 
270,  271,  272,  275,  304. 

Siemens,  William,  194. 

Siemens  and  Halske,  272. 

Siemens's  unit,  262. 

Sine  galvanometer,  234. 

Sing,  Ph.,  121. 

Siphon  recorder,  234,  276. 

Siren,  136,  281,  283,  284. 

Sixteenth  century.    See  Renaissance. 

Sky,  colour  of,  188. 

Smee,  A.,  221*. 

Smee  cell,  221. 

Smithsonian  Institution,  178, 190,  239. 

Snell,  58,  76*. 

Socrates,  9. 

Sodium,  discovery  of,  215. 

Sodium  lines,  155,  156,  157,  162,  166, 
182,  185. 

Solar  heat,  171,  178-180. 

Solar  spectrum,  155,  157,  161,  163,  170, 
171,  178,  181,  185;  place  of  maxi- 
mum energy,  178,  179. 

Solutions,  theory  of,  218. 

Somerville,  Mrs.,  154. 

Soraerville,  Martha,  154. 
gorge,  A.,  284. 


Sound,  among  Greeks,  10-12;  among 
Romans,  15  ;  17th  century,  81, 97, 98; 
18th  century,  136  ;  19th  century,  141, 
146,  167,  176,  184,  279-285;  beats, 
136,  283-285;  Doppler's  principle, 
167;  harmony,  283,  284;  limits  of 
audibility,  281;  overtones,  97,  136, 
282-285 ;  siren,  136,  281,  283,  284. 

South,  James,  105. 

Spark.    See  Electric  spark. 

Sparks,  J.,  120. 

Specific  gravity,  4,  19,  20,  25,  202. 

Specific  heat,  116,  199,  212. 

Specific  inductive  capacity,  130,  247, 
248. 

Spectacles,  38. 

Spectroscope,  166,  167. 

Spectrum,  82,  89,  153-171;  effect  of 
pressure,  166;  analysis,  157-171; 
line  series,  165. 

Spence,  120. 

Spherical  aberration,  84. 

Sprague,  F.  J.,  272. 

Sprat,  47,  286. 

Stahl,  G.  E.,  114*. 

Staite,  W.  E.,  271. 

Stancari,  V.  F.,  136. 

Stanley,  W.,  246. 

Stark,  274. 

Stars,  twinkling  of,  26. 

Static  electricity,  measurement  of, 
128,  129.    See  Electricity. 

Statics,  4,  30,  31,  99. 

Steam-engine,  112,113,206,207. 

Stearns,  J.  B.,  274. 

Steinheil,  K.  A.,  273. 

Stevens,  W.  Le  Conte,  186. 

Stevin,  Simon,  30*,  35. 

Stewart,  B.,  159. 

Stokes,  G.  G.,  162,  163, 188,  275. 

Storage  batteries,  222,  223. 

Striated  surfaces,  141,  168. 

Strings,  vibrating,  11,  12,  97, 136,  279, 
282. 

Stromer,  110,  111. 

Stromeyer,  200. 

Sturge,  Mrs.  G.,  40. 

Sturgeon,  220,  234,  240*. 

Sturm,  J.  C.  F.,  259,  281*. 

Submarine  cables,  234;  telegraphy, 
274r-276. 


INDEX 


321 


Sulzer,  J.  G.,  133. 
Summational  tones,  284. 
Swan,  J.  W.,  271. 
Swift,  49. 
Symmer,  R.,  118. 
Sympathetic  vibrations,  136. 
Synesius,  6. 

Table-turning,  250. 

Tait,  P.  G.,  53,  208,  210,  252,  265. 

Talbot,  W.  H.  F.,  157*,  182. 

Tangent  galvanometer,  234. 

Tartini,  G.,  284. 

Tatum,  235. 

Taylor,  W.  B.,  62,  274. 

Telegraph,  232,  273-278. 

Telephone,  276-279. 

Telescope,  reflecting,  85,  104,  105 ;  re- 
fracting, 26,  37-41,  63,  84,  85,  103, 
155,  156,  167. 

Temperature,  of  incandescence,  159; 
concept  of,  195;  low,  200,  201,  203, 
204 ;  analogy  to  potential,  260. 

Terrella,  44. 

Terrestrial  magnetism,  22-24,  44-47, 
94,  197,  227,  260,  261;  number  of 
poles,  94,  260;  earth's  magnetism 
and  the  sun,  261. 

Tesla,  N.,  273. 

Thalen,  R.,  169. 

Thales,  8*,  14. 

Thenard,  295. 

Theodoric,  King,  15. 

Theophrastus,  8, 10*. 

Thermochrose,  173,  174. 

Thermodynamics,  206-214;  first  law, 
209 ;  second  law,  208. 

Thermo-electricity,  229,  268,  269. 

Thermometer,  89-93, 105-111, 171,  173, 
193-195,  212. 

Thermopile,  173,  174,  178,  243. 

Thilorier,  201. 

Thin  plates,  colours  of,  86, 141,  184. 

Thompson,  B.  (Count  Rumford) ,  115, 
140,  189*-193,  303. 

Thompson,  S.  P.,  236,  240,  270,  277. 

Thomson,  J.  J.,  251,  266. 

Thomson,  W.  (Lord  Kelvin),  24,  62, 
162,  163,  195,  207,  208,  210,  211,  213, 
214,  234,  259,  262,  267,  275,  276,  290, 
293. 


Thorium,  rays  emitted  by,  267. 
Thurot,  C,  4,  25. 
Thurston,  R.  H.,  112,  113,  206. 
Todhmiter,  I.,  251. 
Topler,  A.,  118,  267,  268*. 
Topler-Holtz  machine,  268. 
Torpedo,  132, 134. 
Torricelli,  50,  63,  64*  65. 
Torricellian  experiment,  64,  66,  70. 
Torricellian  vacuum,  96,  264. 
Torsion  electrometer,  130. 
Torsional  elasticity,  131, 
Tourmaline,  128, 147,  267. 
Tourmaline  tongs,  147. 
Tracy,  de,  226. 
Transformers,  244-246,  258. 
Transverse  vibrations,  146, 147. 
Trigautius,  82. 
Trowbridge,  127,  128,  299. 
Truesdell,  W.  A.,  6. 
Tuning-fork,  282-284,  285. 
Turbines,  6. 

Two-fluid  theory,  118, 131, 137. 
Tycho  Brahe,  29,  47. 
Tyndall,  J.,  93,  142,  148,  174*-178, 190, 
210,  236,  239,  282,  292,  303. 

Ubaldi,  G.,  31. 
Ultragaseous  state,  265. 
Ultra-violet  rays,  172,  181. 
Undulatory  theory  of  light,  80,  81,  86, 

101,  137,  140-148,  156,  157. 
Uppenborn,  245,  268. 
Upper  partial  tones,  282. 
Uranium,  rays  emitted  by,  267. 

Vacuum,  50,  51,  61,  6^-66,  68,  96,  112, 
113,  121,  132,  153,  171,  192,  264-267; 
nature  abhors  a,  25,  63,  65,  66. 

Van  Depoele,  272. 

Van  der  Waals,  202. 

Van  der  Willigen,  160. 

Van't  Hoff,  J.  H.,  218,  220. 

Vaporization,  heat  of,  116,  199. 

Varenius,  58. 

Variation.    See  Declination. 

Varignon,  3. 

Varley,  C.  F.,  246,  267,  270*,  276. 

Vaschy,  62. 

Velocity,  of  light,  77-79,  148-153,  182, 
of  sound,  97,  280,  281. 


322 


INDEX 


Verdet,  E.,  143. 

Very,  F.  W.,  179,  180. 

Vibrations  of  strings,  11,  12,  97,  136, 
279,  282;  longitudinal,  280;  tor- 
sional, 280 ;  composition  of,  285. 

Vibrator,  Hertz's,  256. 

Vinci,  Da,  27,  31,  48. 

Virchow,  R.,  139. 

Vision,  theories  of,  7,  19,  185-188. 

Vis  viva,  52,  53. 

Vitellio,  26. 

Vitreous  electricity,  118. 

Vitruvius,  4,  5,  15*. 

Viviani,  36,  64*,  90,  150. 

Vogel,  H.  C,  168. 

Voigt,  W.,  162,  284. 

Volt,  unit  of  E.  M.  F.,  262. 

Volta,  A,  100,  1.33*-135,  219,  267. 

Voltaic  cells,  135,  219,  220. 

Voltaic  pile,  134,  215,  220. 

Voltaire,  56. 

Von  Kleist,  118*,  119. 

Vortices  (Descartes's),  54-56. 

Voss,  Isaak,  76. 

Voss,  J.  R.,  268. 

Vowel  sounds,  282,  283. 

Wagner,  J.  P.,  245. 

Wall,  122. 

Wallis,  J.,  51. 

Walsh,  J.,  132. 

Wand,  T.,  208. 

Water,  25 ;  specific  heat  of,  212 ;  elec- 
tric decomposition  of,  135,  215 ;  ve- 
locity of  light  in,  149-151. 

Watson,  120, 126. 

Watt,  James,  112*,  113, 116. 

Watt,  unit  of  power,  263. 

Wave-lengths  of  light,  156,  168,  170, 
180,  181 ;  unit  of  length,  184,  185. 

Wave  theory  of  light,  80,  81,  86,  101, 
137,  140,  148,  156,  157. 

Weather-vanes,  10. 

Weber,  E.  H.,  280*. 

Weber,  W.,  139,  257,  261*,  262,  273, 
276,  280. 

Webster,  A.  G.,  299,  303,  305. 

Wedgwood,  J.,  111. 

Weight  and  acceleration,  2 ;  and  mass, 
53, 54 ;  of  air,  63-66, 68  ;  of  bullets,  75. 

Weiller,  L.,  183. 


Welch,  W.  H.,  289,  295,  296. 

Wells,  W.  C,  205*. 

Wernicke,  A.,  79. 

Wessely,  K.,  6. 

Weyrauch,  J.  J.,  209. 

Wheatstone,   C,  157,   231,  232*,  233, 

243,  270,  274,  275. 
Wheatstone's  bridge,  232,  233,  250. 
Whewell,  13,  25, 49,  51,  56, 146,  249-251. 
White,  A.  D,  31,  32,  41,  127,  287. 
Whitman,  F.  P.,  186. 
Wiebe,  194. 

Wiedemann,  E.,  7,  18,  19,  26. 
Wiedemann,  G.,  139,  292. 
Wilde,  H.,  269. 
Wilhelm  von  Moerbeck,  26. 
William  III.,  94. 
Willigen,  v.  d.,  160. 
Wilke,  J.  K.,  116,  128. 
Wilson,  B.,  128. 
Wimshurst,  J.,  267,  268. 
Wind  map,  earliest,  95. 
Winkler,  J.  H.,  118, 119, 122. 
Winthrop,  J.,  127,  189. 
Wireless  telegraphy,  256. 
Witelo,  26. 
Wittstein,  20. 
Wohlwill,  E.,  90,  91. 
Wolf,  R.,  31,  36. 
Wollaston,  89,  153*,  154,  172,  181, 185, 

219,  236. 
Wood,  J.  G.,  10. 
Work,  52,  53. 
Wren,  C,  51,  57,  85. 
Wroblewski,  S.  v.,  204*. 
Wiillner,  130,  164, 165,  220. 
Wurtz,  296. 

X-rays,  266,  267. 

Youmans,  W.  J.,  127. 

Young,  J.,  151,  153. 

Young,  T.,  87,  100,  131,  136,  138, 140*- 

147,  168,  171,  175,  185,  193,  214,  280, 

284,  303. 
Young-Helmholtz  theory,  185, 186. 

Zeeman,  P.,  166. 
Zeller,  E.,  12. 
Zipernowsky,  C,  246. 
Zucchi,  N.,  85*. 


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